I actually know of a way that there could be a cheaper transponder system but I just dont know how to go along doing it so i'm gona throw it out for anyone to use.
all across the US thiers toll roads and they all have some sort of EZ-pass type deal that's esentially a RFID tag mounted near the top of your windshield as you pass under the tower the thier system is told that you passed the tower at such time and deducts the money from your account or whatever.
Now imagine the same thing but the tag is mounted in the top of your body you pass over the loop and it reads that your buggy has passed just like a regular system and the RFID tags would be CHEAP i mean thier in alot of the electronics you buy at walmart so you could have one for each and every body you have or slap em somewhere on your chassis