[shaunsteig]

for rule of thumb pricing, I use the following for commons:
-- PSA 3: $30
-- PSA 4: $50
-- PSA 5: $90
-- PSA 6: $160
-- PSA 7: $400 (much more variability as the grade gets higher)

SGCs, I've found, tend to sell for ~10% less on ebay. But I've also found that the difference from PSA shrinks at the BST here and at the third-party auction houses. (speculation -- perhaps the more knowledgeable collector appreciates the SGC a bit more?). Another site for market prices is vintagecardprices.com (disclaimer -- I have no affiliation).

remember to "buy the card, not the holder", as prices will differ within the specified grade depending on the visual quality/appeal of the card. also, the final cost to the buyer will differ depending on the context -- ebay sellers usually then add on a shipping cost, whereas the majority of cards listed on the BST here will already include the shipping cost, whereas auction sites will add on a buyer's premium (typically 12-19%).

Hope that helps,
--S