Commissions - how much to charge?

Hey - I thought I'd get some peoples' opinions about this one. I'm starting to get people (here and elsewhere) asking me if I do commissions and would consider doing stuff for them; apart from the obvious worry of not being good enough to make whatever it is they end up asking for, the big issue I have is that I haven't the faintest clue how to work out how much to ask for something like a commissioned sculpt

So...any and all advice would be welcomed. There are a few things that people have asked me for that would actually be quite interesting to make, but as busy as I always seem to be I don't want to spend ages making something for someone for a pittance. But then again, neither would I want to overcharge...