How much is a 2000 South Carolina gold quarter coin worth?

It depends.

If the coin is thinner and lighter than a normal quarter you may have a "lamination error" that happened when the outer cupronickel cladding separated from the copper core. A lamination error can retail for $15 or so.

If the coin is the same weight as a normal quarter, it's probably been plated by someone with a knowledge of high-school chemistry. Then it's an altered coin worth 25 cents.