I was talking to Mike Masinick at CT and he refered me to a website that has a whole lot of information of sold prices on graded cards. Baseball, Basketball, Football, Hockey and Boxing Sports Card Value Price Guide based on prices realized in auction. FREE Email alerts on pending auctions at no cost to you. VintageCardPrices.com
The bad thing is the get the prices its like $13 a month, but if I am serious about investing a bunch it is worth it. I will probably go with a whole lot of vintage cards and just get a bunch of them on the cheap on auctions and then hold off on 1-2 years and hope that I can make 10-50% on them.
Here is an intersting Mantle auction. PSA 4 but it looks like its PSA 2 1952 Topps # 311 Mickey Mantle PSA 4 Rare High Number - eBay (item 190268354156 end time Nov-27-08 13:30:59 PST)