On your last point -- whether a FOSS project is set up to be able to do anything useful with $800 varies a lot, I think. If it's something put out by a consulting company or by a one-man-band independent or even by a student, sure. But a lot of FOSS projects are either worked on mostly by people on a big-company payroll (in which case their time is spoken for already and the bigco doesn't care about your $800), or else hobby/spare time stuff for somebody whose day-job is unrelated (in which case they're in it for other reasons and preferring not to spend four hours of their free time pair-programming with a stranger is an entirely reasonable choice).
Basically unless you're already making a living by the accumulation of a lot of that kind of $800 transaction, the hassle involved in accepting just one of them as a one-off makes it not worthwhile (tax questions, potential conflict of interest with existing employer, dealing with people as paying clients rather than as people one can simply say 'no' to, etc...) And there's a massive chasm between "maybe once in six months somebody offers you $800" and "enough money coming in to quit the day job".