Have you searched for the cards on Ebay? If you haven't, I'd go ahead and start there, but click on the "sold" or "completed listings" link to get a ballpark figure.
My advice would be to start a few bucks higher than that to gauge his interest.
"For me, Baseball is what made me. They say the first thing a wolf cub believes it sees is its parent. Perhaps that is the way I sense Baseball within myself."