Respectfully, you have a too-reasonable hunch of how decisions are made at the executive level, and because of that, you're reaching an incorrect conclusion.
The primary factor driving the decision making process is not cost but risk.
Many fail to remember the lengths to which companies like Microsoft, Oracle, Sun and others went to create FUD around the adoption of OSS in the public sector. It involved lobbyists, marketing campaigns and a whole certifications industry.
When a CIO selects a technology, they aren't just weighing its merits and price tag; they are actively looking to leave a glowing legacy while actively doing everything they can to avoid being humiliated if the technology fails. See, the CIO knows that, as far as their board of directors is concerns, it's not the software that failed but the executive.
That's why enterprise support packages exist; it's an insurance policy and a promise that if something goes terribly wrong at 2am, someone is going to pick up the phone when you call.
Companies with expensive tools have an obvious interest in convincing their best customers that the competition in inherently risky.