I will guess 35k.
The opponents DO matter. The occasional fan will go see a game if the opponent interests them, pisses them off, or are very good. So yes, being in CUSA does affect attendance.
What I think has effected the overall attendance a lot is the tailgating/parking rules. While the new roads around the stadium have improved the flow of heavy traffic, it also eliminated a buttload of parking spaces closer to the stadium. (like along the side of the road) No where along Sun Bowl Dr can you park in the street. Now the closest parking lots are "reserved" and mostly go unused. The close Handicapped parking lot stays mostly empty even for the Texas game. Since a lot of those folks use the shuttle anyway, why not move that lot farther away?
You can't really tailgate in a UTEP lot. Can't block a spot using a tent. Have to tear down by gametime. Gustapo walking around barking out orders of what you can't do.
UTEP has taken the fun and convenience out of a college football game, thus killing attendance in the process. You can't have a crappy boring team, AND make it a hassle to go have fun, then expect good crowds.
BTW....run, run, run, punt offense IS boring and no one aside from Kugler likes to watch it.