Arrch:
For my set-up, I have an Arduino with Ethernet connected to another atmega chip w/ a HopeRF transceiver acting as a base station. The lights have a small custom board with an ATTiny84, transceiver and a latching relay. After the initial base station cost, it's about $10-15 per node. The latching relay is double pole, so I use that to determine the status. That's been the cheapest I can get it without running wires.
Could you share your schematic?