I would favour the RFID approach. We use these tags heavily for traceability purposes and they are not as expensive as you think. Besides the costs will become much lower as major retailers are demanding that goods are supplied with RFID tags attached as standard.
As for a device then consider the timbatec ruggedised PPC with the RFID reader add-on. WIFI as standard - they're effectively an iPAQ 5000 series. Check the specs as they are enhanced.
If you're worried about theft then just put an RFID tag inside each unit and use standard RFID antennae's on the exit's.
