Good comments above. Sleeving is a big hit in price. Proof . . . important, but more so with more expensive guns.
And IMO, it depends on why the gun's no longer in proof. Chambers punched but not reproofed, I'd say that's a fairly big deal. Out of proof because of an oversized bore . . . if you're looking for a shooter (lower priced gun) and the barrels still have plenty of wall thickness, that one is less critical to me. But more important on a Purdey than on a Birmingham nonejector.
The Brits in general are not as nuts about original condition (reblue, re-CC, stock redone) than are American collectors. As Ted said, that's considered more a matter of preventive maintenance over there--which they tend to look upon, with their guns, more like we do with vehicles (and less like we do with guns, which quite often is not to take them to a gunsmith unless something is broken). If the work's been properly redone and in the British fashion, to me it shows that the previous owner took proper care of his gun.
