I do radon testing within 15 miles of downtown Durham (based on Google Maps). The standard cost is $100. I use state-of-the-art Corentium Pro Digital Radon Monitor by Airthings, which allows an accurate reading with an instantaneous report once the two-to-three-day testing period is complete. For clients who are also using me as a home inspector and have a home with a partially or fully finished basement, I offer this service for $50, because the likelihood for radon problems is higher with this type of home and I want to strongly encourage you to get it tested. For homes beyond 15 miles away, I can recommend other companies for you.
If you need more information on what radon is and why you should consider testing for it, please visit the EPA's radon web page.