Sam's club is selling a battery
powered CO detector that has a digital readout for less than $20 USD. (That is really cheep)
I have used this model for years and what I really like about it is that it will store the CO level and you can access the memory to see if it has detected any CO and that feature really gives me peach of mind. (Minute qnty of CO)
It will automatically alarm should the CO reach a dangerous level.