The Five God Dragon (better known as the Five-Headed Dragon) is worth approximately $15 dollars. It can only be found in the structure deck Dinosaur's Rage, which consequently costs about $25 dollars.
If it does not say on it "LIMITED EDITION" and/or if it has a silver or no hologram in the bottom right hand corner then it is a counterfeit, and is only worth its artistic value.
Hopefully yours is not! The best cards to use with this card are Dragons Mirror, Future Fusion, and Shooting Star Bow - Ceal for a direct attack of 4000!