Like most of the other posters have said, it's supply and demand.
Twenty years ago you could find SKS's by the crate for 80-100 bucks each. It's not that they were that bad of rifles, but that overwhelming supply drove prices down. Now that imports of them have really slowed, their cost has gone way up, and they've pretty much reached an equilibrium price of about $400. Owning one, I'd say that's fair. They're worth it, but I wouldn't pay much more for one.
I think a similar thing is happening with AKs. Stocks of surplus don't last forever, and AKs aren't as plentiful as they used to be. Now that that glut is gone, they are finding their way to a fair open-market value. As supply decreases further, prices will only go up.
That being said, I think their price is also kind of tied to the AR - generally speaking, they are a substitute good for buyers looking for a military style semi-auto rifle. I doubt that a WASR type AK will ever be priced much higher than an entry level AR, though they are pretty close right now.
In a similar sense, I think that there is an over-supply of ARs right now. If I had to bet, I would say that AR prices are more likely to go up $100 in the next five years than go down $100 in that time.