I just thought I would mention to you that in our area...WI and MN there is a huge difference in what is considered an "acceptable" level of Radon...although I do not know the level for MN...the level here in WI is an average of over 4.5...some states have acceptable levels as high as 14! So you may want to be careful in stating that across the board levels should not be above 1.
The reason for this difference may be our soil types in the Northern part of the states are loaded with granite subbase...the shifting in this base is what can cause trapped radon gas to escape...etc. As a for instance...just North of my home there are properties that test out with Radon levels of 55 and higher!
Penelope, you must refer to your states regulations to determine if this can/will be an issue. If the level is above what is considered by your state regulation, then you may have a "material defect". You do have the right to request that the sellers correct the problem via a proposed amendment to the sales contract. They of course do not HAVE to do anything (it certainly would seem to be in their best interest because as was pointed out before, now they do have the duty to disclose this defect).
Mitigation systems in our area run about $1000 for a 1700 square foot ranch...costs may increase if you are dealing with a larger or two story home.