They may not be accurate but they don't need to be at the levels where CO becomes fatal. It is better to have several inexpensive ones than just one or two $300 highly accurate ones. Since you don't know where the CO will be localised.
You can test them yourself fairly inexpensively look for "HOME SAFEGUARD HO-CO2 Carbon Monoxide Detector Tester, Aerosol." Get a Ziploc bag or similar air tight baggy. Place the alarm in a bag, spray the Aerosol in it, and wait. It should go off in 2-5 minutes. Obviously it is best to do this test outside.
Consumer Reports also tests them:
http://www.consumerreports.org/products/carbon-monoxide-alar...
If you can get a good, trusted brand for home CO monitors, I'd say go for it.
Apparently, Kidde is a trusted brand in fire alarms etc. in the US, so I think this one ($20) is probably equally trustworthy:
https://www.amazon.com/Kidde-Battery-Operated-Carbon-Monoxid...
I shudder to think that you would risk your life on a device of questionable quality.
The problem is surely knowing whether the expensive equivalent is just the same internals in a different retail pack (with other 'quality' signalling). That's genuinely hard for something that needs a lab to be tested.
There are a few companies that sell low level CO monitors for residential use. They're all somewhat pricey.
A CO monitor will set you back $235.
$235 sounds (relatively) expensive.
I recently replaced the 3 CO alarms in my flat - they cost me around £23 (29 USD) each [1].[1] http://www.toolstation.com/shop/Electrical/d190/Carbon+Monox...
Don't use a propane heater, please. It's dangerous to you and potentially to others nearby.
Source: Been there.
I use First Alert personally (they have model with Z-Wave integration that ties into my home automation system). We carry a few Kiddie smoke/CO detectors on the fire truck to install if we notice a home doesn't have one (or if the one they had failed).
For those who haven't seen it, here:
https://www.draeger.com/en_aunz/Applications/Products/Mobile...
But if you want to do it right, you also need a calibration station and calibration gas bottles. It is quite an effort.
[No product placement, also look for Honeywell and others.]