I got a small Carbon Monoxide Detector in Homebase recently. It cost me all of €12 as it's a model that displays results only. I have it placed beside my gas boiler and check it everyday.
For an alarmed detector, I could have paid nearly €40, and as I've only 1 boiler in the house which is well ventilated, along with excellent vents in every room, I didn't see the need to purchase a full alarm.
Again I could have purchased this in Homebase also, although I've seen them in Woodies, Atlantic and also B&Q