agreed, 10-15 percent, but it must be a whole lot more to really pique the interest of collectors/dealers.
First of all it has to be off-center and I don't believe that is an off-center. I think it is probably a misaligned die. If a coin IS an off-center it has a premium value no matter how small the percentage is, but the premium value rises the further off it is, up to a point. Usually once they get past 75% off the value starts going back down. Also a dated piece is more valuable than a non-dated piece. A misaligned die can also have a premium value but only if part of the design is off the coin.