You didn't mention parallel users.
If you're handling 10 parallel users, ANY modern open source database would be a good choice. Interbase / Firebird is a nice small fast database that's easy to maintain.
If you're going to handle 1000 users, you'd likely do better with Postgresql than MySQL.
But you didn't mention parallel load so I don't know which to recommend. Oh, and Postgresql is 100% free, period, no matter how you use it.