If all you want is communication from one helmet to another, seated on the same bike, the cheapest solution is the chatterbox HJC-50 plus 2 headsets, a total of about $130. While the chatterbox radios blow, the simple wired systems actually work quite well. They are not voice-activated, basically they are just always on. The unit acts as a small amplifier, connecting the microphones in both headsets to the speakers in both headsets. For best results, use earplugs and turn the volume way up on the units; this blocks almost all wind noise and communication works right up to highway speeds. The HJC-50 has a rechargeable battery, but tends to last a full day of riding before needing a recharge.
http://www.hjc-chatterbox.com/hjc50.htm
For some higher quality equipment, look into the Autocom products. I've got their Active-Plus model on my RT and it works incredibly well, at higher speeds and with better sound quality than the chatterbox units. It also can plug into bike power, and the unit itself can be mounted somewhere on the bike (mine is under the seat, with the two headset jacks peeking out the side). Autocoms have some pretty neat features that allow you to link in music, radar, gps, and even cell phone. When you start to speak, it lowers the volume on everything else so you can talk easily; when radar goes off it lowers everything else immediately, things like that. It's pricier, but not as bad as people seem to think. The full system goes for $300 including 1 headset; an additional headset is $75.
http://www.autocomamerica.com/active_plus.cfm