Re: I read this as propaganda
That works better with the Oracle example than it does with the Microsoft example. The difference is that in the case of Oracle, there's still a substantial amount of stuff specifically developed for Birmingham's particular needs by someone, whether that's Oracle themselves, some other business using them, or Birmingham's hired programmers. On that, I agree with you; calculating costs is very difficult for both approaches and trying to claim that one is easy and the other isn't is unconvincing.
But if we're looking at something like Office365, there's a lot less code being written for the individual user. People already know most of the features involved in this, what they cost, and, if they're already using 365, whether they need them. Figuring out how much it costs to add Intune for device management is mostly a licensing question with some calculation of how long it should take to make the policies that will be enforced. Compare that to developing or adapting an open source device management system. The costs for that will be hard to determine, because if you are building it for the new government Linux distro you're also creating, you'll have more things you need to build in but more control over the system so more ability to integrate them during development, whereas if you have to manage fleets of lots of operating systems, you have to develop many different clients and deal with OS makers breaking things you relied on. That is, in fact, more difficult to calculate and it's likely that, if you only consider finances, it's more expensive. A proper comparison needs to also compare the expected features, maintenance cost, risks, and all those complicated things which makes both prices, but especially those for the system that doesn't exist yet, hard to calculate.
Another reason this is hard is that we're not just comparing two alternatives. We haven't decided whether this should be a single government body making all the software they no longer want to buy, a process where other businesses are asked to do it and sell the result to the government, a process where businesses are given cash and asked to make the result open source, a process where existing software is supposed to be used unmodified, or separate methods for different parts of the government. That makes the calculations hard, and although they're well worth doing, many of those who would have to would prefer not to.