MSSQL isn't foss; you pay what it's worth to you as part of the licensing. That's one of the issues at play here, paying for software licenses versus using foss components to build your own saas, and then donating something to the foss maintainers.

A simpler way to say this: What's the average compensation for a developer, and how many hours do you think it would take for one of your developers to build some small foss component? I bet it's more than the $500 donation.