Two headed quarters can be found on many online web sites for prices ranging from two to ten dollars. The price is low because two headed coins are created by fusing two coins together. They are not the result of a minting error, which would make them extremely valuable. Many people do collect these coins nonetheless.
A two headed quarter is considered to be the same currency as the typical American quarter. Rather then having a head and an eagle the design is just altered. This same idea is seen in those commercials advertising alternative 9-11 twenty dollar bills.
I think that it should be worth more than a regular quarter. I think it should be worth at least one dollar, although some people would probably pay more.