Well, I don't know if other places do the price trick, too, but I have seen it myself in the 4wp ads. I didn't believe it until I saw it for myself first hand, but comparing two of their mags a few months apart, one with the 3 for 4 deal, sure enough... prices were higher in the one.
SoCal- You may want to consider your two options again. Yes, the MTs probably are a bit cheaper, but the ATs will last at least 50% longer I bet. Plus, stronger side walls (I believe the regular PC MT is only 2-ply while the BFG AT is 3-ply... probably depends on the load range, though) and better street manners have to be worth at least the additional $100. I'd love a set of some kind of MT's for the X, but unless I stumble on a set of spare wheels, I'm sticking to AT's on the daily driver.