I was thinking about buying a used french horn, but it's had the lacquer removed. I was wondering if any of you knew how much it would cost to refinish the french horn.

Why do you want to relaquer the horn? Other than make it look pretty, laquer only offers a nominal protection for the horn from corrosive materials (like acid from your skin). However, when the laquer starts to flake off the acids will make little pits in the metal (think miniature pot holes) which is actually worse than even wear.

If you like the way the horn plays, adding a layer of material (laquer) on top of the brass will change the response of the instrument and possibly the timbre.

Remember, it doesn't matter how the horn looks, it is how it sounds. Don't worry too muc about laquer. I have three horns 1 with laquer, 1 with out and one that the laquer is badly flaking off. I think rather than re-laquering the third, I may just strip it or just wait for the rest to fall off.