Paratroopers are hard to value. The reason why, is because most people don't even know what they are, and buy into the fake hype about them.
In reality, they are just surplus military rifles, that have had the barrel cut down and sometimes refurbished. There is really no added value to them, over what the basic gun is. There's not collectiblity as a specific model.
So it really comes down to condition and if it's all matching. Basically, look at it as if you're looking at the original rifle. Anything that is not correct or has been replaced, drops the value.
This is how an SKS collector/enthusiast would look at it. Now factor in the uneducated buyers, and you've got standard paratroopers selling on Gunbroker for $600-$700.