Most valuable toy: Pikachu Illustrator Card ($20,000+)
Original price: $10-$20 (per pack/deck)
Selling for now: Free-$20,000
The rarest Pokemon cards have held onto their ridiculous values as the original players have grown up and moved out of their parents' basements. The rarest card, the Pikachu Illustrator card (only six were printed), sells for tens of thousands of dollars each time one of them goes on the market. The Pre-release Raichu Card, Tropical Mega Battle Prize Card, and the First-Edition Charizard Card command thousands of dollars each as well. There are a number of other cards that go for hundreds in mint condition.
Before you search through local yard sales for deals, understand that the most valuable cards have to meet a very rigorous set of criteria. Not only are the most valuable cards first edition, but they must also pass a strict grading test from PSA (Profession Sports Authenticator). The exact edition and condition of a card is so difficult for a novice to determine that eBay has published a detailed article on the subject.
The photo above is from that article and depicts to nearly identical Charizard cards worth vastly different amounts.