I have Snap-on, NAPA, CP (Chicago Pneumatic) , Campbell Hausfeld and Ingersol Rand. The NAPA is probably the best impact wrench for the money, but still not cheap. I woud say my old...OLD CP impact is the best just because it is affordable and still works after the beating it has taken all these years. It has since been retired to home duty in my garage.
If you have any pawn shops around, might be worth checking. Craigslist and yard sales also come to mind. You can probably find a very good one for $10-$25
This one is dirt cheap and might get the job done: Harbor freight impact $21
I might be tempted to try this Sears Craftsman impact $50
it looks very similar to the Ingersol Rand gun. I haven't used the plastic bodied wrenches though. Not sure how much abuse they might accept. Looks like it might have more torque than the Harbor freight cheapie. Sears looks to have a couple more choices under $100 that look reasonable too.