How about something like this from banggood?
http://www.banggood.com/Wholesale-Security-RFID-Proximity-Entry-Door-Lock-A=
ccess-Control-System-10-Keys-p-61575.html
$US10 with ten fobs

Or US$5.70 for this one
http://www.banggood.com/MIFAIR-RC522-Chip-IC-Card-Induction-Module-RFID-Rea=
der-p-81067.html

NB: I haven't read into these in the slightest so not sure if they'd do
what you want, I just recalled seeing cheap RFID systems

Lee

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.