Test kits are often cheap, like $10-$20. But then most companies charge you additionally to get the results. So often then the cost is actually around $50. The EPA recommends long term testing, and testing in different rooms in different seasons. Radon levels change constantly. You might have a radon level of 20 Becquerel in one room, and 120 in another. One room can be 30 Becquerel in the summer and 150 during the winter. If you want to measure multiple rooms and homes (like those of family members) it quickly becomes cheaper to just buy a continuous monitor.

Not true as they can't be calibrated. Continuous radon devices should be calibrated yearly.