It's small, and on a normal business strike coin, I wouldn't give a care. But it's on a proof coin, which is supposed to be perfect. Another thing... Is it worth anything? It's a die crack just above the word liberty. Any input is appreciated.
Assuming that's really struck on the coin and not damage, it looks more like struck-through-lint than a die break. It would be unusual for a crack to open up entirely within the field of a die. I can't comment on it's value, that's not in my wheelhouse. Others more knowledgeable will chime in.
That's a small Strike Through. They are common on Proof coins. Usually lint or a fiber. Nothing major. No premium for that.