Reliable UHF RFID when tags are on metal or liquid surfaces is a solved problem. See Omni-ID's tags for an example, albeit not for only a few cents, as the meta

Reliable UHF RFID when tags are on metal or liquid surfaces is a solved problem. See Omni-ID's tags for an example, albeit not for only a few cents, as the metal makes good RF performance a little more challenging http://www.omni-id.com/on-metal-rfid/ http://www.omni-id.com/on-metal-rfid/