Approximatly how much does it cost to repair a turn signal switch on a 1997 Chevy Lumina when the center brake light comes on but the two main ones remain off?

This more than likely has nothing to do with the turn signal switch. Check the bulbs, fuse, and look for loose wiring.
Actually, I had a similar problem on my 97 Lumina and it was the turn signal switch. I'm getting it repaired tomorrow so I'm not sure what it costs yet.