We (the D language foundation) have a bounty system.

It's not bad, however I wouldn't expect the world from it: the kind of things that get bounties put on them are often not $150 bugs - i.e. it's not always the incentive you might imagine it to be.

The bounties do however work as a statement of intent for new contributors to see what is important vs nice to have.

I also would let the bounties be set by users rather than centrally.

We also maintain a fund ourselves with enough money to fund a few things ourselves, our so-called HR fund, I recommend doing that also.