I think what thoffman is trying to say is the $400 carbines are not USGI--if you want a real USGI, which I assume you do (much higher quality, history, etc) make sure you look up the contractors who made them and purchase one of those. Now that the CMP is out of carbines, they are harder to find and prices have gone up a bit. You used to be able to get a nice one from the CMP for around $500, but not anymore. Keep an eye out and depending on condition expect to pay $500-$700. Carbines with early features will sell for more. YOu might get lucky and score a deal for less than that though.
the 400 dollar ones are not original war pieces. "fakes" if you will.