[cnlwsu]

xbee radios are ~20$ although there are some more expensive ones if you need 40 mile range.

[stephengillie]

The only ones I've been able to find are around $100~$300.

[mmetzger]

Some kits may be over the $100 mark, but individual radios are usually in the $20-$30 mark.

https://www.sparkfun.com/search/results?term=xbee&what=p...

https://www.adafruit.com/index.php?main_page=adasearch&q...