If its a GI gun, even in not so great shape, $400 is a good deal.
Ive bought 5 GI Carbines in the past couple of years, and the cheapest one cost me $800. The average price around here for one in decent shape is probably around $1000-$1200.
Function wise, all 5 were around 75% as they came with GI mags. I had to rebuild the bolts on all of them, replaced the recoil springs, and swapped out the GI mags for current Korean mags and once I did that, they are all around 95%. I still usually get one or two stoppages every 100 rounds with any of them.
All my guns had been back to the arsenal at some point, and had the adjustable rear sights and bayo lugs. For some reason, the rear sights on all 5 had been knocked out of their staking and were loose. One actually fell off the gun while I was shooting it. Easy fix though.
The front sights on three were not rezeroed when they were redone though and needed filing down to get a proper zero with the markings on the rear sight.
The later bayo mount/stock clamp is the better design and way to go.
Even though they were updated at some point, only two of mine have the mag catch that is meant for the 30 round M2 mags.
The Korean mags are cheap (around $12 for the 15 rounders) and both the 15 and 30 rounders work well. USGI mags are hit and miss, mostly miss.
Numrich/Gun Parts has pretty much anything you need part wise, and at fairly reasonable prices for GI parts, to get them running right if you do have problems.
This will give you an idea as to what goes for around $1200+ here.