@mvexel self-test kits are indeed $15 but generally need 3 months (and the "closed up 3 months" of winter) to work. Then mail in and wait a month or two for results.

I tested that against the AirThings radon detector which gives a real time reading as well as an averaged one. In the same location over 3 months, they were about 10% different. More expensive, but can be shared amongst family & friends, and gives much faster readings. Also helpful when testing mitigation effectiveness.