This is one reason why I'm going to be providing bounties for more than just security bugs. Even if people don't expect to find security bugs, they might find a typo in a comment and earn themselves a $1 tarsnap account credit -- and as demonstrated here, simply looking at source code can result in finding bugs even if you weren't originally looking for them.
Clever. Especially since "normal" bugs occasionally have that nasty habit of turning into security relevant bugs.
It will be interesting to see how close you can manage to get something resembling good review on a budget. Hopefully other people who are in similar low margin code businesses will keep an eye on your experiment to see how it works out.
Thanks for being so open about how you're trying to make things work. I hope you'll be publishing all the awarded bounties? (I suppose I should just wait for your follow-up entry.)