This depends on a lot of things, we'll need more details...
Headliner or opener?
Who's covering production? (sound, lights, etc.)
Is there a Promoter? (Radio Station, etc., advertising?)
Where the tickets sold through a ticket agency? (like Ticketmaster, Metrotix, livenation, whatever)
Everybody needs to get their cut, and it can be substantial. The venue, production (sound, lights), promoter, ticket vendor, and then the bands need to split, and the headliner should get the biggest cut. Sometimes there is next to nothing left for an opener.
We usually do flat rate only. But when we've done small (350 seat) venues where we got a cut of ticket sales and there were NO other bands, we got 80% of the sales after tax. Now if there were promoters and other bands, we'd get far less, which is why we generally don't do that.