Not much I guess. It seems to be in terrible condition and there doesn't seem to be anything that makes it stand out like autographs or rarity, but I could be wrong. It might still be worth something because of it's age though, not much, but something.
Btw this thread just reminded me of pawn stars, love that show
Edit: I did some research and found
this. It looks just like your poster. It seems to be a poster that came with the vinyl soundtrack back in 1977. More info
here.