If it were in 100% original condition (stock, bayonet, 10-round fixed mag, etc.) then this sounds like an okay price to me (not great, but okay). To me, the after-market stock and 20-round mag reduce the value (unless the original parts are also included). If you like the configuration, then you'll probably enjoy it, but I wouldn't jump on it thinking you are getting an outstanding deal.
I bought a like-new Russian and Chinese Paratrooper a couple of years ago for $500 total. I figure the Russian was worth a little more, but I wanted the Paratrooper (16") and they aren't too common around here. This spring a friend and I bought Romanian SKSs in very good condition (much better than average) for about $130 each plus shipping and FFL fees. These are the same configuration as the Russians - though not as sought-after. A local pawn shop now has a nice Russain for (I think) $395. They aren't known for giving great deals. I suspect that $350 would take it easily.
Doug