Figuring on how much more traffic the shop will get and how much profit they can make on that, probably $50 is fair at this point. But since they asked you to do it, go for $100 and a scone.
I played in bands in high school & college, we would get hundreds of dollars a night, back in the 70s & 80s. But that was for an established band at weddings, clubs, etc.
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Current inventory:
F22 & G05
Cadillac CT5-V Blackwing w/3 pedals
Past: E24,E28(3),E34,E36,E37,E38(2),E39(4),E46,E89,E92 (obviously),F01,F06,F10,F30,F87,G12,G30(2),G82(2)