Aleeady pointed out, but wth...
1. Communication on trails. Helps with straglers, wrong turns, and easy spoting / quick comms.
2. Highway information and entertainment. If you haven't passed time on a road trip listening to truckers, you're missing out. Hilarious stuff. And it can tell you when to go ahead and take a lunch break instead of running into the gridlock from that accident that just happened.
3. Weather alerts. NOAA equipped radios can be had for 40$. Best reason to have one, bar none.
4. Alert trucks / other jeepers when entering / leaving trails or technical / blind sections. You do not want to surprise a logging truck.
5. Emergency comms, though this better be your backup to your backup for breakdowns in a populated place only (or an orv park).
They are anywhere from 20$ to several hundred. They all transmit the same wattage, regardless of microphones, antennas, or any other accessory. 4w by federal law, most are <3.5w. Some people juice them but beyond being illegal, it only makes you transmit further - it does nothing for recieving.
Install is easy. You need the radio, a cable to connect the radio to the mount, a mount, and an antenna. Some add a spring or quick disconnect for flexibility or irregular use, and it is highly recommended to get a swr meter for cb bands and tune your system.
If you do rely on your radio as an emergency device (or just really like it), look into getting a HAM ticket. The test is not as hard as many people think, the radios are much cooler and more feature packed, it is possible to talk to space or the other side of the world, and a decent mobile rig can be bought and installed for ~ 300$, though it is highly addictive and the highend units do cost more than I paid for my Jeep. By over double.
Great place to whet your appetite - http://www.arrl.org/