When the USO sponsors a tour date the band will get travel money from their last show to the USO show at the gov't rate($.41a mi per vehicle) lodging for all band and crew, plus a per diem for each band member and crewmember(anywhere from $50 a day to $100) depending on the city. If it is a MWR show, then it is a pay for play and they get their guarantee, plus and back-end that is in the contract. It used to be that MWR had a tremendous budget for shows like this, but now it is about a tenth of what it used to be, so when they do get a show they have to charge or go belly up. And you are right they do have a built in audience, example: FUEL played Fort Bliss in Sept, Ft Bliss population-45,000 soldiers(not counting family members) attendance at the show- 26,000 at $10 for non-military $5 for military, Free to Wounded Warriors. Of the 26,000, 19,000 were military(one division was in IRAQ so that cut down on military attendees. The show made money, the band made money, the local bands made some money.
There is a good reason for band to hook up with The USO or MWR or United Forces Entertainment, it's called exposure. Once you get your foot in the door, you get on a list of entertainers and maybe asked to do shows overseas. The band ASKA from Dallas has been on USO tours to the Far East(Japan, Korea, and such) 5 times, Europe 4 times, and to Kuwait and IRAQ once. IT pays off in the long run.
Just a little insight into how the USO and MWR works, they are great organizations that really help our
Soldiers, Sailor, Marines, Airmen, and Coasties.