Quantity   Component name
3 × Arduino Uno, $10 PIck one of the Arduino Uno clones with a 3.3V/5V switch, and use it only in the 3.3V mode. I've used the "Buono" with great success, and you can find it readily available on ebay. Then buy a real Arduino Uno just to support the project :)
2 × RFM69HW, $4 915MHz wireless transceiver. The HW (high power) version has the most range, over 700 feet without any complicated antenna design. Find it on ebay or anarduino.com. Use the 915MHz version for US/Canada, and 868MHz version for EU.
1 × Wiznet 5100 shield, $8 Ethernet shield for the Arduino ethernet gateway. Buy the fancy version if you got the cash, but the cheap ones seem ok too.
1 × Raspberry Pi, $40 Get your pie.
1 × For battery powered units 1) MCP1702-3302E/TO, voltage regulator, 3.3V, good quiescent current. The other components are just your run of the mill capacitors and oscillator.