This is just a guess, but I think they are cheaper because the 12x is too much magnification for a 42mm glass and the 12x42 probably does not sell nearly as well as the 10x42, so the price is lower to help move the inventory. The 12x moves into the long distance and tripod mounted "Big Eyes" category and at 42 mm does not have enough light in the system to compete with 50-60mm glass.
