That stand alone unit costs around $50. I also have a few of these (see pic) around the house that are connected to my alarm system which was $100 each. They also detect smoke. Building code requires a smoke detector in each bedroom, a detector within 10 feet of every bedroom door, one in kitchen, living area, and one close to your mechanical room. And one CO monitor on each level of the house. They all have to be connected to each other. So if one detects smoke they all go in to alarm and a very nice lady tells you in English and french (voice recording) if smoke or CO was detected and to get the hell out.

But I have the smoke / CO combination installed everywhere where code required smoke only, and then a few stand alone units at various locations. I am paranoid about CO in the house. I have 2 furnaces, a natural gas water heater, natural gas fireplace and a wood pellet stove, and a natural gas radiant heater in the garage that are all sources of CO. They are all vented to the outside, but anything mechanical could malfunction. The vents could also sometimes ice up when very cold outside
I have a monthly fire drill with the kids where we practice where to go and what to take. You may escape the fire but end up in -40C outside in your pajamas and bare feet.