I am not any sort of signature expert - but the poster itself is a Special Edition One-Sheet from 1997, which is maybe £30-£40 quid(ish), whether the signature adds or takes away from the poster value I have no idea.
The one thing I can say is while the line is close to Alec Guinness' from the film, that is not his signature - maybe someone else from that scene or something?
Sorry - not much help.