I've been running a Humble Indie Bundle deal, kind of. Pay whatever you're comfortable with, as long as it's at least 1gil.

The lowest I've gotten so far was 3 gil, with the highest tip being 20'000 gil. Overall, people usually tip 5'000 gil, and I get pestered quite often for melds, so I can hardly complain.

Edit: This is as an Alchemist, by the way.