The prices on a SKS depend on which one you are trying to buy...generally, the Russians are the most expensive followed by Yugoslav and Romanians (the debate on which is better starts right after the Russians lol), Polish, Albanian, Korean, and then finally the Chinese versions.
The price for a good Russian are about $600 to $650, then they generally go down from there. I have seen some more expensive guns listed on GunBroker, but that isn't realistic just yet. Just sellers wanting to drive the market for their own gain.
They are a far more accurate gun than an AK, although the reliability to fire in severe conditions would definitely go to an AK. That's at least what I can tell, I had 3 SKS at one time, a Russian, a Yugo, and a Chinese version. The Russian was the best built, but the Yugo was heavier and seemed to perform the best. The Chinese gun didn't hiccup either (none of them did), but it was pretty worn out and probably needed a new barrel (it appeared to have seen some battle).
That was a good price on that AK of yours, they seem to be climbing daily. Both the AK and the SKS could be had for a fraction of what they are selling for now just a few years ago, but that's the same for most guns now.