I once played a charity gig for extensions to a local church. It was for a dinner and dance at a local 5 star hotel. We went in for £1600 and got 'knocked down' to about £400 after the organiser laying it on thick. We found out tickets for the evening were £100 each. There was an auction of promises after the meal - it went on and on and on. We had been there at 5 to set up.
The Auction made £20,000.
We finally went on at 11:30, played for half an hour! That was about £60 each for 2 hours travel, 2 hours set up and pack down, 6 and a half hours sitting around and 30mins of playing.
After that we were very careful to do our research for all gigs.