There are a few diffrent types of SKS's and here are some of the price ranges I have seen them go for. In general SKS are very good rifles and are a lot of fun to shoot.
There are ones Chinnese ones made by Norinco. $150-$275 range These are good one, haven't here of to many problems with them.
There are Yugoslavan one with the grenade lancher on it end. $80-$175 range. Some times the gas port on them leak and cause them to not fire right, but if you get one that is in very good condition this is usualy not a problem.
or Yugoslavan ones without the grenade lancher $150-$250 range, these are good ones, haven't heared of to many problems with them
There are Russian ones $250-$400 range, always a good chose, just about all of them are good, and good quality
Romanian ones $175-$250 range. Don't know much about them (I don't have one yet)
Albanian ones $90-$250 range, another good one.
The Mausers are just about always good. There are many many diffrent kinds out there so you would have to narrow down what kind it is (usally what country it was made for). Then I sure that someone that knows more about Mausers then I do could help you out.