cURL would operate such a program in good faith, and quickly earn the trust of the people who submit the kind of bug reports cURL values.

Your bank would not. Nor would mine, or most retail banks.

If the upfront cost would genuinely put off potential submitters, a cottage industry would spring up of hackers who would front you the money in return for a cut if your bug looked good. If that seems gross, it's really not - they end up doing bug triage for the project, which is something any software company would be happy to pay people for.