I agree with most responses you are receiving. Radon is fairly common around Denver and the foothills, and it is typically easily mitigated to a level below EPA recommended action levels (4.0 picoCuries per liter). Mitigation systems usually involve a fan that creates a negative pressure pulling the radon from the ground and venting it outside the house. Because a fan is mechanical, it has a life span and will need to be replaced at some point. The radon mitigation company I use typically installs a whole mitigation system installed for about $750 and provides a 10 year warranty. I do not know what fan replacement costs.
If you are using a buyer's agent to represent you (and you absolutely should), your agent should have provided you with the resources to understand radon and its mitigation. I have a useful link below. I would also suggest that your agent help you understand the common occurrence of this in our area, and its insignificant impact on the salability of a home. In addition, opening a dialogue with the sellers would be a wise choice rather than terminating the contract if it was a home you wanted. The sellers may have felt it was not applicable since the radon levels were mitigated. The listing agent does not complete the Seller's Property Disclosure, so they would not have been advised regarding identifying the system. I would advise my clients to ask the sellers why the system wasn't identified before becoming concerned about other items not being disclosed. Also, keep in mind that the Seller's Property Disclosure is a helpful document; however, the inspection and due diligence are how you do you research and make your decisions.
Radon tests are common in real estate transactions in Colorado (because radon is common), and a continuous 48 hour radon test typically costs about $100. That would assure you that the installed system is working and the home is within acceptable levels.
Hope this helps!