It's a matter of keeping two parties happy and viable. One answer is to request a minimum that will for sure cover expenses and a meal for the dog. Then split the balance of the take with the club. So:

50 people x 5 pounds = 250 pounds. Less the 100 pounds guarantee leaves 150. Split two ways that another 75 for the performe(s) and 75 for the club. The club also keeps any other cash it takes in via sales of coffee, tea, alcohol, etc.

I got screwed that way a few times, but sh#t happens. Chalk it to experience and let everyone you know about it. Clubs with a stirling reputation--The Yellow Door in Montreal was (is) such a place--will survive. Less trustworthy places will go down the tubes. There is some sort of Darwinian process at work.

Best of luck to you all.