How much does radon gas cost per gram?

You can't buy it.

The most stable isotope of radon has a half-life of about 4 days (contrast this with the first element most people think of when they hear the Word "radioactive," uranium, where the kind used in nuclear reactors and atomic bombs has a half-life of seven hundred million years, and the most common isotope has a half-life of about five billion years).

Most people who need radon ... and there are very few ... generate it in situ from a solution containing radium that costs about $6,000 per milliliter, with each milliliter containing about 15 picograms of radon at any given time. A gram of radium produces about 1 mm3 of radon gas per day, so to get a gram of radon per day, you'd need about 101 kg of radium. Radium is not commercially available in those quantities, so the ultimate answer is that it's literally priceless; you cannot buy a gram of radon at any price.