When we were playing lots of gigs, I'd tell them our basic rate for anything up to two hours and then how much to add for other stuff, like travel out of the city, bringing a PA, wearing any kind of a costume, learning specific songs, or extra time by the hour. We had a list of fees and they could spend as much as they liked to customize their experience.

I used to try to "qualify" the gig to determine if I should charge more or less, but that is difficult in many situations, and often has to be done on the fly during a phone call. As soon as I started quoting the same to everyone my life got a lot easier.