These things aren't cheap and the batteries are not serviceable; prices haven't fallen much below $50 per unit for most cars (Hondas, for some reason, have less expensive sensors but they aren't interchangeable with other makes). The batteries last about 7-8 years in service, so all of us are looking at at least one replacement cycle if we keep our cars. :surprise:
If you get sensors from TireRack, apparently you have to go to the stealer anyway to get them programmed to the car - each time you change between sets. I haven't heard of any independent who has been able to access the system (the ATEQ tools apparently don't work on the new Impreza). The best answer might be cloning the IDs of your current set (some sensors can be set that way) if the dealer will give you those numbers; then you could switch back and forth without a problem, but again I don't know of anyone who has done that with one of our cars (yet).
Please let us know how it goes!