Just thinking out loud here--what about just using RFID for most of the pallets, and then putting powered electronics w/GPS only in some fraction of them? Say, 1 out of 20 or 50 (or whatever) pallets would have a battery, GPS, and an RFID reader; then those could read the RFIDS on any nearby RFID-equipped pallets. And transmit their location and the nearby RFIDs back to home base.