Is there any way to know that an expensive CO detector like this is worth getting for home use, instead of one costing 10x less?

Hard to say, but Omega definitely care enough about their brand and reputation to have a good QA system and testing of products in place. Since testing that a CO monitor is working is very hard for a consumer, you rely a lot on trust in the brand.

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...

Anecdotal: I've bought cheap versions of things online enough times that I don't want to do it again (just got home from helping a friend install a battery that didn't even fit properly). I've had a few things melt and a couple explode..

I shudder to think that you would risk your life on a device of questionable quality.

>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.

A CO "monitor" is permitted to alert at low levels of CO (10-20 ppm). A UL-listed code compliant CO alarm may not. I personally think this is silly.

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...

Are there any that work in temperatures below freezing? I'm living in my car and using a propane heater when it gets cold.

Wow. That is definitely a recipe for carbon monoxide poisoning. I encourage you to find some other way to survive, because it's unlikely you'll survive long doing that.

If you're able to get a small ceramic heater, you'll find that there are many unexpected places to plug it in. E.g. near gas station car wash exits, some parking lot lamp posts, etc. Check with the owner for permission, there are some that will grant it, especially if you're able to offer some lot cleanup or other bartered assistance for them.

Don't use a propane heater, please. It's dangerous to you and potentially to others nearby.

Source: Been there.