Fishey

To do an alignment right takes time usually over an hour on most machines if you are doing a full alignment even if the adjustments are easy as hell to reach and use. If you have a super badass machine (Reflector/zero runout) it might only take you slightly under an hour to do but also keep in mind that these machines cost upwards over $20,000+ and they don't pay for themselves.

Sure, you can get a $100 alignment at alot of places but your alignment is going to be garbage and your car will handle like garbage. Most techs at firestone/pepboys/tirediscounters know nothing about car setup. They pick your car from a list on the alignment machine and try to hit specs and usually do so without doing any of the work that needs to be done prior to even having an alignment done. (Ballast, Tire Pressure, Check balljoint/bushings/bearings/rack)

Also, your rear tire wear might not have anything to do with rear camber it could also be rear toe. Are you getting any chopping in the tires? If so your problem is certainly toe if not it could still be toe just to a lesser degree. There are plenty of people who run far to much rear toe-in and as a result the inside edge of the tires try to roll over as the left and right try to push the car in both directions at the same time. Anything over .30 total and your going to start noticing it with some negitive camber in the car.