Sometimes, I’d have my company pay $50 to merge a pull request :-)
Most of the solutions proposed in this thread for one-off bounties here have already been implemented for years, which is why I am convinced that (with the current level of funding), bounties don't work.
When I find an issue on GitHub requesting a new feature or reporting a bug that is important to me I should be able to throw down $5+ bucks to incentivize someone to fix it. I also know that I would personally contribute to OSS more if it didn't feel like I was doing community service but rather actually getting paid for my time.
A person who wants to pay for a bounty would need to submit an issue, and the maintainer estimate it's complexity. At that point, the person filing the issue could click "Buy this issue" if the maintainer has bandwidth.
Again, not easy -- and I'm sure the UX / business / execution would be delicate.
If you want that, you probably should look at UpWork instead of GitHub. Bounties are a nice way of prioritizing things, but most OSS projects won't get the kind of OSS contributions they want for the kind of money they need, simply because the kind of contributors they want can get 10x better pay on the job market, so why they should bother with OS when they can freelance?
Discounting the recent trends of companies open-sourcing stuff to build a market for themselves elsewhere, open source was always, first and foremost, doing a community service. This aspect is why OSS developers put time into it instead of getting extra paid work.
It would still be nice for those who are interested in working on those issues regardless of payment.