I have a funky little program [thanks to an amazing friend who needed to test some things out for a client project - ponies and I were group guinea pigs!] that does something on a much much smaller ikle little scale for ebay prices but the problem is tracking them all! It really only works by asking it to search [current and ended sales] and then generate an average.
Although the shopping sites can work [sometimes and in a limited way] for searches...it would be very interesting to have something running constant price checks!