First of all it's called the "Butcher" cover not the "Butcher Block" cover. As the previous poster mentioned there is a lot of information available if you search a little.
The album exists in 3 different levels of collectibility:
Stage 1- an original "Butcher" cover released by Capitol.
Stage 2- an album that was recalled (actually all of them were recalled) and returned. Upon return to Capitol a new cover, the white picture with the Beatles standing by a trunk, was pasted over the "Butcher" cover and re-released to the public. If you have one of these there are a couple of places where you can actually see the original cover underneath. I believe one of them is around Ringo's head but the details are available as I mentioned.
Stage 3- an album whose pasted cover has been removed by steam or chemicals. The "steamed" covers are quite easy to spot as the "Butcher" image is grainy from the steaming. The chemical removed covers are tougher to spot and generally in better condition.
Precise figures are not available but estimates say there are 6,000-60,000 legitimate "Butcher" albums in existence. The stereo version is always worth more than the mono version in comparable condition.