The commercial variants have been bringing good money for many years. Naturally, how much money depends on which model, as some are less common, or more desirable than others.
The SKS-M model (remember, there is only one actual model called an M) usually brings in the $650-$750 range in great shape.
The SKS-30 (mistakenly called the D) has been selling for over a grand for years now. About the lowest you might find one, is around $800. But most SKS-30s sell for over a grand nowadays. I sold mine about 3 or 4 years ago, for $1200.
Actual SKS-Ds (made by Midwest Ordiance) sell for about the same money, but less people know what they actually are.
Most of the other AK mag variants sell for good money because they were less common, and more desirable to collectors. (SKS-NR, SKS-63, KS-30, 88 and 89 Sporter, SKS-93)
Naturally, being collectible firearms, the value goes up if the original box and accessories are there as well.