I guess I'm blessed. With the exception of 2 very nice Japanese bikes in the early 80s, and a handful of very nice Britbikes in the many years since, I have ALWAYS bought cast-offs, non-runners, basket cases, and a few lots of non-runners/parts/basket cases at the very bottom end of the price range. Most of those have been nickle-and-dime'ed together slowly, as scratched-together-funds slowly trickled in. Then, over time, they got a few new bits added, eventually new tires, and finally some of them got decent paint and an occasional new seat. It just so happens I made it a business for a dozen years, so was also to take advantage of volume discounts on parts, paint, powdercoat, chrome, wheel lacing, cad plating, etc. Sending a few of my bits out along with one or more client lots to various tradesmen worked out quite well to slowly improve my collection.
I've been able to take advantage of selling when demand has been reasonably high, and hardly ever forced to sell due to financial crunches apart from major restorations that were undertaken with the sole purpose of selling, so I sold. Really, a good few sales were forced by lack of available space for more bikes.
In all this time, I'm far ahead as far as the bottom line, but I suppose full analysis has to account for the fact that I did depend on much of those profits feeding the family... Still, 50+ years of riding and a net gain with a very nice collection still in the garage(s) is out of the ordinary.
Bottom line, on topic, I can sell below the current market on my Interstate, sell a bit below or at market on the green VR880, and get a very tidy sum on the Dreer monoshock prototype bike (there is no "market price" for rare bikes, it's down to a niche buyer), and I'll be just that much farther ahead. The monoshock bike isn't going anywhere for the forseeable future anyway...