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 <a target="_blank" rel="noopener noreferrer" href="http://anarduino.com">anarduino.com</a>. 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.