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It's a hard question.
Part of the answer is that it depends on what you can afford.
But why give to this particular FOSS project and not others? I would like to see some kind of "whole system" donation framework where I could donate to "all the software I use" in a way that was "reasonably" distributed. But I don't think there is anything like that, not yet anyway.
Anyway, it makes sense to donate both to maintain existing software, and to support entirely new things.
@eliasr @neil @lightweight
If projects use liberapay you can do something like this
https://liberapay.com/