Walmart offers the First Alert Home Radon Test Kit, model no SC07, for $13.97. (Amazon charges the same with free shipping for Prime members.) Opt for in-store pickup to dodge the $5.99 shipping charge. That's the lowest price we could find by $6. This 24-hour test checks for the presence of radon. The lab testing fee is included in the price.
Of note, Lowe's has it for $12.97 in select ZIP codes with free in-store pickup.