I seem to have a pretty odd scale. I do a ton of tourist group work and school groups.For tourist work I get $6.00 (US) per head with a 20 person minimum. The concert lasts about 1.5 hrs. If I have o travel, I get $0.35 per mile for anything over a 50 mile round trip. Since most tour busses hold 45 to 55 people, I love this work!!! I have a maximum of $600 per hour for larger groups.
For school groups, I get $250 for a half day (1-3 hrs) and $450 for a full school day. I do ask for a lunch break and at least 10 minutes between classes to preserve my voice. For travel, the same rule applies as above.
I do pretty well with these rates, I perform about 75-100 dates a year and don't seem to have any complaints about price. It fits in well with my day jobs too!