A Plainfield is a commercially made carbine, and I'm
no expert on those, but I know enough that not
all the parts will match a USGI carbine should you need to replace anything...but the Plainfield (early models anyway) is pretty close from my understanding. Many of the early runs of post-war commercial production models used USGI parts, but didn't as time went on, Universals for example.
I did find this
page if it helps.
Offhand I'd put these in the $300 ballpark, but someone with a much better knowledge than mine will chime in at some point. I think I'm probably close to it's worth with that figure, but I can't comment on it's quality because I don't know enough about them and haven't shot one. My estimate is based on what I've seen at gunshows in my area.
Problem with M1 carbines today is the prices on them have gotten pretty high and you can end up shelling out $600-$700 dollars for a USGI M1 but it's likely to be a well used rifle, or you could buy an inexpensive commercial model that's not going to function well. I have a Rock-Ola from '44 that was purchased for $25 45 years ago, it would now sell well north of $1000 because it's in excellent condition. Auto-Ordinance does make a current model to USGI specs that compares pretty well to the originals. I have one and while, it's not as good as my 'real one'...it's better than any other commercially made model I've seen. But be warned, it needed to be sent back for headspace work after I bought it. After that its run perfectly.
It is possible to find a USGI M1 still in good shape for less than an AO, but it does require some looking and will be more expensive than a Plainfield or Universal. CMP, the Civilian Marksmanship Program, used to have them at a reasonable price...but I'm pretty sure they're out of stock and not likely to get anymore.