Trumpy, the basic detector sells for about $25, whether it be a battery powered or plug-in type. Fancier ones, with features such as a continuous LED read-out, can cost as much as $80

Many products are offered that have both a Co detector and a smoke detector in the same housing. These typically sell for about $50.

The sensing element of these detectors is, I am told, somewhat "biologic" in nature, and thus the limited life. Some models have sensing cartridges that you replace, rather than the entire unit. BTW, these units also alarm when they freeze.