Depends on the condition and company that produced them. Some of the Upper Deck Griffey rookies are worth a ton in good shape. Iciro usually has a decent market in my experience and you will usually fetch book price on his cards. I have had some luck selling some Pujols Topps rookies that I bought back in the day for an average of 15 bucks each.
The Hall of Fame player cards are usually decent sellers and you can always sell them book price to someone. Though not if your looking to sell to a dealer. The PED players are not selling well right now. No one wants to say that they just love Barry Bonds or Sammy Sosa just yet.
If your looking to sell on E-bay you might do pretty well. If you sell them in bulk to a dealer your looking at about 25-50 percent of their listed price in general.
Good luck.