How can I detect the presence of another specific item electronically cheaply (I can’t afford a RFID reader)?

I am attempting to create a system that remotely detects another unique item electronically without having to purchase an expensive RFID tag and reader. Additionally, I believe a proximity sensor can only detect the presence of ANY item rather than a specific and unique item. Also, if possible I would like the solution to incorporate compact items that need to be placed on a bicycle and a keychain. Any ideas would be greatly appreciated. THANKS!