This answer is covered in our publication, Building Radon Out -  on page 22.

Soil testing for radon is not recommended for determining whether a house should be built radon-resistant.  Although soil testing can be done, it cannot rule out the possibility that radon could be a problem in the house you build on a lot.  Even if soil testing reveals low levels of radon gas in the soil, the amount of radon that may enter the finished house cannot be accurately predicted because one cannot predict the impact that the site preparation will have on introducing new radon pathways or the extent to which a vacuum will be produced by the house.  Furthermore, the cost of a single soil test for radon ranges from $70 to $150, and at least 4 to 8 tests could be required to accurately characterize the radon in the soil at a single building site.  Therefore, the cost to perform the soil testing is very high when compared with installing the passive radon system in high radon potential areas (see the EPA Map of Radon Zones).

Why not wait to install the radon-resistant features until after the home is completed and a radon test is performed?

It is much easier and far less costly to prepare the sub-grade to improve the soil gas flow before the slab is cast.  Also, the pipe for the radon-resistant system itself can be run more easily through the house before it is finished.  This significantly improves aesthetics and can reduce subsequent system operating costs by planning to route the pipe through warm space to maximize passive operation of the system.

The best way to determine the radon level in a home:  Test the home for radon after occupancy.