Kensal, sleeving goes beyond being "non-original". Were that the only issue, then a sleeved gun would not have much difference in value than a gun with a set of replacement barrels--which is also non-original. The difference in the two is that if replacement barrels are fitted--a much more expensive undertaking--then more careful attention is likely to be paid to overall balance. Sleeving, as Dig pointed out, can run the gamut from a fairly cheap process (certainly compared to new barrels!) to a good bit more expensive process. Not that there aren't sleeved guns that balance well and are very good jobs, but it's certainly not a given.
And a sleeved gun is only superior to an "original" which has some sort of serious barrel issues: badly pitted, really thin walls, etc. And the process will likely only be done on an original which is otherwise in pretty good condition, and which would have sold for halfway decent money were it all original. In other words, no one's going to sleeve a hard-used BLNE for resale, because the price it will bring (assuming a good sleeve job) won't justify the cost of the sleeving. But it can be a good way to own a gun you otherwise could not afford (like my Purdey 16, described above) for way less money than an original in proof and in reasonably good condition.
