Asking prices for Yugo M59/66 SKS rifles are averaging just under $400, with the average sale price a little over $300 lately. Here's data: GunStockMarket Yugo M59/66 SKS data
The real story lately is not so much that prices have increased dramatically, but that the supply is trending down. Take a look at the general SKS tracking data
at the same site, which shows the prices holding steady overall, but the number of available listings beginning to drop.
Even with a recent set of Chinese SKSs (imported from the Balkins is the theory, specifically Albania), and these new imports (yes, direct imports from China have been banned for a long time, hence the third-party country as the source) which are still not C&R being of Chinese origin originally, the supply of SKS rifles is tapering off.