Give your local real estate office
a call. They deal with Radon inspections all the time and know which companies are reputable and offer the service at a reasonable price.
Also if you need to remediate radon in the air or well water they can give you the names of a few companies that provide those services in your area. Compare prices and the actual equipment to be used including maintenance costs if you do buy a system.
Check out this site also:
http://www.des.state.nh.us/EOH/Radon/