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Lots of people do this as a hobby - getting paid for a hobby is a very good way to make it not fun anymore. If its consistent $$$ that might be worth it. For a one shot thing, maybe not as much. I think it also can be difficult to mix volunteer stuff with money. Posting in a bug tracker that multiple people work from which anyone can take on a bug from, that you will do it for $x, can make it feel like you're using a…
Sure, but for almost everyone, there's a dollar figure at which they're willing to stop having fun and start making money. Maybe that dollar figure is $1,000/hr for some open source maintainer out there (I bet it's much lower for most of them). Why not put a line in the readme which says, if you want to commission a specific change, send me an email to discuss, my consulting rate starts at $XXX/hr. Or there's a $YYYY…
IMO one of the best ways to contribute to OSS while making some living is to do product development and/or consulting work, and structure it so that you can methodically encapsulate and open up the components involved. Of course, this unfortunately does not apply to every project out there, but I believe Django was born and had been developed this way for example, as well as probably many well-known projects that we don’t know the precise origins of.