I live in one of the more irradiated parts of America, El Paso County where our countywide average score in residences is in excess of 4.00 picocuries/litre, where http://www.epa.gov/radon
indicates a responsible individual needs to take action.
For the most part, the real estate community in Colorado Springs is pretty well informed about it, and as a general rule of thumb, I tell buyers to test for it ($125) during their inspection period, no matter where the home is, what kind of home (condo), anything. The rule seems to be more aggressive, not less aggressive, and I would not rule out a day in the future when it is a disclosure nationally and treated like Lead-Based Paint.
Saying that, your question is about resale. What is totally amazing to me is that while Colorado Springs is fairly conservative, we are extremely progressive in dealing with Radon. It's everywhere, you can mitigate it to a "safer" level, move on with life. A $1200 repair is usually less than 1% for dealing with something the EPA claims is the 2nd leading cause of lung-cancer in the US.
But in Summit County (like Breckenridge and Keystone, where it can be really high) the agents I've met treat it like no big deal and may disparage someone taking action. Same thing throughout parts of Metro Denver. There are people who will say "well, the other homes in the neighborhood don't have a system... is n't that a defect unique to that house?" The logic is okay on the surface. Deeper down, the reality is: have any of these other people tested for it? Did their inspector scoff about it because they didn't have Radon training? Did someone bring up the fact that the threshold is much higher in Canada? Or lower in Sweden? Is the house in Canada or Sweden?
It's one of the crazier things out there, and I think you as the buyer are in the driver's seat. This is the time to get the present owner's cooperation in making remedy. Hard to do that after you close. But I think for your peace of mind you can consider a county like El Paso County Colorado, home to a half million people. and our average score (according to both UCCS Chemistry Lab which does most real estate Radon tests and El Paso County Health Department) is in excess of the EPA level and what you are finding in the home you're buying. In fact, what you're finding is low by our standards! But we deal with it, we move on, and most homes around here have habitable basements.
A really good idea would be to ask for an extension of a couple of days in the inspection period to get written bids by one to two EPA-qualified/certified professionals. Evaluate how much of a visual eyesore they need to create to get rid of the problem. That's really the biggest concern. Most systems can be put in a discreet location, and since it does it's job, it really moves on to an "out of site, out of mind" reality.
The one thing I would disagree with in the previous comments is the pocket incidence. Yes, some areas generally have higher radon, but that doesn't mean you only search for it in some neighborhoods or that there is any consistency to the scores within neighborhoods. We see 800% variances on the west side all the time (3 in one home, 24 in the next, 56 in the next, 5 in the next, all perfect tests). We have a neighborhood out east built on sand which around here usually does not have any radon. I still have tests done because the fill dirt for some homes (not all, just a few) came from the west side of town, which is all the shoulders of Pikes Peak, which is all granite, and granite has uranium which decomposes to radium which decomposes to radon. No joke, the non-native west side fill dirt imported radon around the foundations of some homes thirty miles east. So you never know. The houses are fine, and once a mitigation system is put on, the air quality is probably better than most other homes.