My ceilidh band has, nominally, a fixed fee, which I quote, e.g. x? for 3 hours of music with 5 musicians and a caller. The fee goes up if they want to go on after midnight, and/or if the gig is outside a 30 mile radius of my house.

Should the client want the use of our PA before or after the dance, then one member of the band undertakes to 'babysit' and gets a personal fee. Some functions also like an acoustic trio, and we negotiate the fee for that, depending on distance, times, etc.

Friends and our personal causes get a 'mates rate'.