What is the value of a Joe DiMaggio rookie card?

Answer:

The 1938 Goudey Joe DiMaggio is considered his rookie card, and sells for about $4,000. and up in excellent+ condition. The same card graded by PSA in near/Mint-Mint condition could sell between $15,000. AND $40.000.
A good way of finding the value of collectible items is to see what they are selling for at auction. eBay would be a good yardstick for this. Regardless of what an item is listed for in a "price guide" the true value is what collectors are currently paying for it.