@neil
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.
@lightweight