My general rule is that, "Everything in Real Estate is negotiable." Whenever I represent a buyer, I always expect to re-negotiate the price after each inspection. High radon levels are unhealthy and can cause many illnesses, including lung cancer (a friend of mine purchased a house in Arizona, he came down with lung cancer 2 years later, radon tests show elevated levels). As for who negotiates, you can do it yourself, your Attorney, or you can hire an experience Real Estate Professional. Since home purchases include large sums of money, I recommend you use a professional experienced in Real Estate negotiations. More importantly, you need to discover the source of the radon and have the problem treated and resolved before closing. Good Luck.