To qualify for a mod being paid I'd probably set these as requirements:
- Fast, Helpful Support
- Regular Updates (by regular I mean not every year, maybe every few months at least)
- The modification is of considerable size (code wise)
- The modification adds a lot of features (not something that is just minor)
- The modification is open-source (Paying for something you can't see isn't very wise)
- The modification is written well (Bad code = bad mod)
That's all I can think of right now