Let me get this straight. -- You're going to save 30 cents per stud using 2.5" -- You're going to spend time and $$, putting wire behind your studs to hold the insulation in place.
I'm in the middle of the same project. I found the difference in cost to be less (in the neighborhood of 20 cents), but this only adds up to about $10 extra for a 20' x 20' room. If you're concerned about the insulation touching your foundation wall, why not use the poly-wrapped insulation?
Besides, fiberglass doesn't 'wick' water like cotton, so why worry about a gap between the walls?