I used to collect cards religiously 10-12 years ago, mainly basketball. I used to have a bunch of Shaq rookie cards, but I sold them all off to some dude like 7 years ago or so. The thing was, I could never get anything but 50% of the book price for them from any kind of serious dealers (card shops and shows), and most of them were just a pain in the ass to deal with, they didn't care about some punk kid trying to trade with them. A few years after I stopped collecting, I ran into someone that wanted my lot. I think I got like $250 for it, not bad at the time, even though it was much less than book value of everything (but there was NO way I was going to get full book for anything anyway).
I still have the 1989 complete Upper Deck set, with Ken Griffey Jr's rookie card, so I guess that is my most expensive. Looks like a good condition griffey rookie is going for about $70 on ebay, I remember when it was worth $125.
Oh yeah, I also did those Magic:The Gathering cards for a while, too. That was short-lived.