$50 flat fee no matter if it takes 5 minutes or 59 minutes. my plumber charges $70 for his goons to look at my boilers. my mechanic charges $60 per hour. why can't we?
I charge $35 per hour for my $8 per hour staff's time during wedding setups.
one important point, you are not selling product, so you have to cover other overhead costs with the sale of labor.