What is the value of a 1900 copper nickel?

Except for the part-silver "nickels" minted during WWII, all US nickels have been made from the same cupronickel alloy used for modern nickels.

It's possible your coin has been exposed to chemicals or something else that affected its color, which would reduce its value. Unless it's in top condition, even without damage it would only be worth 1 to 4 dollars so you haven't lost much.