I've added below a few more points that might be useful when considering TCO.  Not all are completely documented, but I thought it might be of help also.

Cost of a Perpetual Use License
What is the price of all Perpetual Use License that you need to purchase.  This is a one-time licenses fee that entitles you to use the licensed software for as long as you like without any additional charges.  Note that this is typically limited to the then current version of the software, and if you want to have bug fixes, updates and upgrades then those typically need to be purchased separately either via a Support Contract or via Upgrades Licenses, or a combination of both.

Cost of a Cloud Service Subscription
What is the price that you pay for the Cloud Service.  Typically this is a monthly or yearly fee that you need to pay, that allows you to use the software.  Typically this then also includes, in addition to the right to use the service, all the evolutions of that software (bug fixes, updates and upgrades), (limited) technical support and  the underlying hosting infrastructure (OS / compute / utility / redundancy / data protection).  Although one needs to be cautious, as it is not uncommon that extra fees apply for new feature sets that are released, or certain constraints may apply that add to the cost unexpectedly (e.g. traffic volume, typically out of the cloud).  And noteworthy is the fact that kind of mechanisms for Cloud Services are typically very convoluted and hard if not impossible to predict (i.e. translating the billing mechanisms and units of dimension to what it will be for your operation is extremely difficult if not impossible without effectively just doing it).

Cost of Updates
Updates typically consist of minor evolutions of existing software features and bug fixes.   These updates might be charged separately, or alternatively be part of an overall Maintenance Service bundle.  For Cloud Services, Updates are typically part of the overall Cloud Service Subscription.

Cost of Upgrades
Upgrades typically consist of major evolutions of existing software features and new functionality.   These upgrades might be charged separately, or alternatively be part of an overall Maintenance Service bundle.  For Cloud Services, Updates are typically part of the overall Cloud Service Subscription, although quite often different subscription tiers apply and access to all upgrades at no additional cost is not necessarily guaranteed.

Cost of Scaling
Cost of Scaling is an important consideration.  First is to identify the different types of scaling that may apply, and to what extend they have an impact on different other cost categories.  This can be quite diverse, and some areas are more likely to affect your cost basis then others depending on the anticipated.   Different types of scaling could include more users, more managed elements, more types of elements, more services, more historical storage, different domains of application, etc.  Impact could be on one or more cost categories, including for example additional licenses, increased subscription fees, additional compute infrastructure, increased storage, etc.

Cost of Technical Support Service
For professional mission critical deployments, you are likely going to require professional support services to assist you where needed with both the administration, maintenance and continuous configuration evolutions of the platform.  Note that various types of technical support services are available, ranging from more community-style / online support all the way to highly individualized support by a subject matter expert, dedicated to your case.  Technical Support Services are typically also subject to different levels of SLAs, which are worth considering in this context.

Cost of 24/7 Technical Support Service
Optionally, depending on the nature of the application and associated business, you may opt to extend your Technical Support Service for continuous around the clock availability.

Cost of Optional Future Components
Often software solutions come with various options.  What is the likelihood of needing some of those options to achieve both your short and long term goals.   In some license schemes, it is not that easy always to understand what options could be potentially needed at some point to be able to accomplish specific goal.  Sometimes options are complete logical modules in the overall suite (e.g. Dashboards & Reporting), which is more easy to anticipate (macro options).  But the most tricky options are the ones that are effectively available as part of the  licensing that you have, but that come with certain feature constraints (i.e. you can use the option, but only to a certain extend and it is more difficult to understand how quickly you may come into a position where you absolutely require the capabilities that are optional) (micro options).  The latter is very typical for Cloud Services Subscriptions, where you subscribe to a specific tier, and where the differentiation between the tiers consists of many scattered small features.  Cloud Services Subscriptions are notorious for being very accessible (e.g. free entry level tier) and quickly bumping you up to higher cost tiers once you are committed to it (e.g. only professional security in higher tiers).

Cost of third-party mandatory components
Sometimes certain software has exclusive dependencies on other third-party software products (i.e. it does not function or it is not full featured without those) that might be subject to license fees, and which are not included in the license fees that you are paying for (e.g. OS, database, etc.).

Cost of underlying hosting infrastructure
For an on-premises installation, irrespective of the type of license scheme, there is always a cost associated for the underlying hosting infrastructure which consists of compute hardware, data storage, utility, networking infrastructure, security, real estate, etc.  For cloud services this is typically, but not necessarily always, included in the overall service charge.

Cost of admin maintenance
Cost of executing administrative tasks and housekeeping such as software upgrades & updates, platform migrations to new compute hardware over time, back-up of the platform, ensuring security of the installation, adding new users (external and/or internal), maintaining security profiles, etc.  Note that a lot of these can be largely impacted by the availability of features such as SSO / LDAP integrations, automated / build-in backups, and much more.

Cost of installation / deployment
Any solution, on-premises but also cloud services, come with a cost for the installation / deployment of the solution.  This can largely vary from one solution to another, depending on the complexity of the installation and deployment process.

Cost of initial set-up & configuration

Cost of architecture

Cost of configuration evolutions
One of the major hidden costs is the actual use of the platform, and its ability to evolve along with the needs and changes, both from a business and operational perspective.   And this cost is only getting higher as the industry evolves more and more towards a platform economy that demands agility, and the ability to respond fast to changes that can come very unexpected.  Pretty much any capability could be considered a simple check box (e.g. fault management - check, ability to integrate third party data sources - check) that can be easily be checked for multiple software platforms.  But if you then look more closely on how that is done, how easily it can be changed and tailored to your specific needs (which might be even unknown to you at that stage), the different software platforms could be very different, and range from being extremely rigid and static to being very open and highly adaptable.  The cost associated with a rigid solution can be exorbitant.  In fact, whereas a single open and highly adaptable software solution might serve you easily for decades, static solutions can have a very short life cycle, resulting in cascaded expenditures for new solutions (i.e. new licenses, cost of deployment, installation, configuration, training, etc.).

Cost of training for system administrators

Cost of training for system developers

Cost of training for end users

Cost of down-time
Depending on the exact area of deployment and the role of the platform, down-time of that platform comes with a cost, and therefore needs to be minimized.  The degree to which you can minimize down-time depends on many things, including specific qualities and characteristics of the platform.  While down-time might be caused due to malfunctions or planned upgrades and updates of the platform, increasingly more attention is required towards evolutions of the platform, as those become a continuous aspect of it.  Therefore it is important that key evolutionary activities can easily be done at run-time, without impacting the operation (e.g. adding & evolving API integrations, developing and changing workflow logic, etc.).

Cost of integration

Cost of loss of opportunity
Lack of functionality

Cost of loss of efficiency
One of the important hidden costs

Cost of time to market

Cost of price evolutions

Cost of lack of product continuity
What is the installed base of the product?  How long has that installed base been around?  What is the history and track record of the company evolving the product with the new technology trends and needs as part of their maintenance programs, or have they introduced such essential evolutions as options that come on top?  Product continuity can have a tremendous impact on TCO.

Cost of lack of provider continuity
Is the product offered part of the core business of the provider of that software?  How much of their business does it represent and how committed is this provider to this product?  Is the company an established value or a start-up, what kind of funding drives the company, what is their financial health and track record?

Cost of lack of security