Rob Sass, author of "Ran when Parked: Insider's Secrets of Buying, Selling and Enjoying Classic Cars on a Budget" has a theory that in general, most cars depreciate in value for the first 25 years, at which point they reach their price basement, and their values start to climb again. (K-car owners rejoice! Your cars' value can't get any lower!) 

Certain cars get to break that rule though...