Alice,Seems to me you could sort this out for yourself by figuring it strictly on the time involved. They say it takes 10 hours of practice for one hour on stage. Say the gig calls for three songs — total of 10 minutes. If you spent a hour and 40 minutes in preparation, you've got almost two hours invested in the whole thing, plus driving time, setup, etc.
Now decide how much your "labor" is worth — and don't short yourself just because you enjoy performing — and charge that as an hourly rate.
I figure skilled labor where I live makes $25 to $30 an hour, so I should be worth that too.
I know it sounds mechanistic, but I think it works out to a pretty fair price.
cheers,
david