It seems that their values for common cards are about right, but their values for stars/HOFers are not accurate.
A helpful site for card values can be
http://www.vintagecardprices.com/. And a simple search through completed Ebay auctions can give you a better approximate value than PSA SMR.
Then again, you know the old adage; It's only worth as much as someone is willing to pay for it.