Carbon Monoxide Detectors do vary quite alot in price. If the best buy was simply a matter of price then the two or three pound spot detectors would win.
However, as can be seen in other areas of this site, the spot detectors, while cheap, are not the best choice.
A battery operated alarm will be simple to install and can be done by anyone and would work out cheaper than having to have one wired in by an electrician.
Another type are plug in alarms which have some of the benefits of a battery alarm without the need to check batteries and change them every so often.