@h4ck3r9, @mike and I have discussed this a lot. We decided to work up to keeping 6 months worth of running costs as a "float," just in case we have bursts in traffic etc. We're then planning to get a half decent donation together and then making it, maybe $100 or similar, to start. As we grow, the donations will likely grow too.
We're getting pretty close to our float now, so I think we can probably start looking at putting money aside for the our 1st donation in the next few months.