The Chinese manufactured several variants of the SKS. Depending on specifics, you may looking at anywhere from $200 - $600+. Paratrooper models and the detachable magazine SKS-M and SKS-D fetch much higher prices than a standard Type 56 carbine.
It depends on what you'll be using it for. The Mosin Nagant is generally more accurate and has a much further range than the SKS. However, the SKS is semi-automatic, smaller and lighter.
Between $250 and $350 for a decent one.
SKS Microfinance was created in 1998.
$150 - $1300, depending on specifics. You could get a non-matching Yugo for between $150 - $250, whereas the much rarer North Korean SKS would be closer to the $1100 - $1300 range.
The D and M models accept AK47 magazines whereas the standard sks has an integral magazine.
The only SKS carbines currently which require registration are the Chinese-made SKS-M and SKS-D models. Those could not be imported into the state after 1989, and had to be registered by 2000.
100-350
All of the fixed ten round mags are cross compatible, but cannot be used in the SKS-M or SKS-D, both of which use a detachable AK magazine.
Very similar, minor differences,
50-500 USD depending on specifics
$100 to over $1000, depending on the specifics.