First some assumptions to make it easier:
1 Band per weekend at $246 flat pay.
Taxes are 0%.
Profit and mark-up is 0% (calculations are made on break even).
No cover.
If a bar is paying minimum wage of $6.15/hr (yes I know wait staff get paid less per hour, but the house is on the hook for tips totaling less than $6.15/hr) and lets say the average band gets paid a flat rate of $246 (to make the math simple

If the wait staff makes their quota of tips, the house is on the hook for just $2.89/hr. This roughly halves the cost of the employee and a band making $246 flat would be equal to just over 2 minimum wage full-time employees.
Assuming your crowd is made up entirely of 110lb women who stay under the legal limit (.08 BAC) and stick around for the entire time between 10pm and 2am, you'll sell about 3 drinks each. If you figure they are all drinking shots and there are 17 shots per fifth, and the stuff they are drinking is $15/fifth. To pay for the band at $246 flat and with 100% of your shot sales going to them until they are paid, your crowd would need to be about 92 strong to pay the band.
About 44 would be required if the crowd were entirely made up of 200lb males, but really who wants to imagine that?