One gets the feeling that, in the USA, Mode S is a stopgap measure and only being required in lower airspace for Part 135 and up. The current goal appears to be get ADS-B working and then think about mandating that for the whole fleet, GA included. So the FAA are not going to make your spamcans fit Mode S. It looks like the Europeans are heading down the path of forcing everybody to fit Mode S without the benefit of the huge volume savings from American sales. Soon after the FAA will require ADS-B and they will become nice and cheap, just in time for EASA to tell the GA fleet to take out the mode S and put some ADS-B kit in instead (which they will then tell you to turn off as it's overloading ATC and airline pilot's screens)