The Breitling Watch Source Forums

I don't think you'll get any agreement on the percentage of devaluation. It's like your guitar example, it only matters to those it matters to. There's a market for everything and that's based upon each individuals opinion of what's important to them. I like patina dials that show the history of a piece, Kurt would throw those in the trash. So if you're looking for a scale of depreciation you're not going to find it because it would still be based upon someone's opinion. As it is now nobody can put an exact value on an unworn, nos (as it left the factory), 806, that everyone will agree on. The best you'll get is a range of value and the reason for that is it all depends on who's interested in buying it at a specific moment.

There is no standard answer to your question and a google search will reveal many such threads elsewhere that might illuminate your thinking.

http://www.christies.com/features/Vinta ... 722-1.aspx
http://forums.watchuseek.com/f20/dial-r ... 40943.html
http://www.iwc.com/forum/en/discussion/28221/