I had this come up back in 2009 when I relocated for work. It didn’t cost me anything, my new employer footed the bills. The remediation method was, as someone else already mentioned, sealing the sump and installing a vent pipe with a fan to exhaust air from the cellar.
The average radon exposure level in the United States is 1.3 pCi/L. The average radon exposure level in Warren County in Ohio is 4.8 pCi/L, with 43% of the indoor air tested being higher than 4.0 pCi/L (The level at which the EPA recommends remediation) with another 25% being above 2.0 pCi/L.
When I sold my home there I had it remediated after the test. I’d have preferred to skip the test, and just remediate without it on general principles, because remediation is cheap, and helps even if the levels are down in the 2.0 pCi/L range, but I wasn’t paying for it, and the relocation company did it their way, test, remediation, retest. The whole thing came to less than $700, and the testing ended up being more expensive than the fix.