Brian's points are quite enlightening! As a framebuilder, he looks at it from a craftman's point of view. I'd love to tour his shop with him when he has such a range of bikes for restoration...
However, even leaving aside rarity, there are several factors to consider:
The bikes we love combine being tools (get you from A to B) with craftsmanship (great or not so great joints, etc.) and art (overall line of the bike, graphics, etc.)
Just like in the world of arts, the biggest names didn't always use the best craftsmanship, and art is full of fads. Even the tool aspect will be up for debate. As long as the bike doesn't break, there are different opinions about what makes a good-handling bike, etc.
Brian mentioned the Japanese who take a Rene Herse for the concept of a complete bike and assign a value to it. The value takes into account the tool (a bike custom-built for a purpose with fenders, racks, lights, etc.), the craftsmanship (hand-made lugs, usually at least good workmanship or better, hand-made parts), and the art (the bikes usually look right, the graphics are - if you like them - wonderful). It is interesting to note that the final value does not exceed the cost of making a new one by all that much - I don't know what a Toei costs, but a new Singer will run $ 4500-5500, and one with special parts from the 1950s may cost almost twice that (you have to provide the parts). Does that mean the Herses are undervalued?
What is surprising, though, is that only Herse qualifies. Singer is desirable, but much less so. Why? Maybe because they are still made? I don't like comparing bikes to cars, but consider that Bugattis for a long time were the most desirable cars. Top sporting Alfa Romeos from the same period are at least as well-built, beautifully engineered and rare. But the name didn't ring as much in the 1960s and 1970s. (Bugatti was gone, and left when his cars were at a peak.) Only recently have Alfa caught up in value. Singers probably will do the same some day.
All other builders seem to interest the Japanese very little. A French friend bought a most amazing Narcisse (or was it a Maury? - I saw the bike only briefly) that had beautiful detailing, workmanship, custom parts. A truly special bike (which I hope to show in my newsletter soon). Very rare, too. It was bought from a dealer who specializes in selling bikes to Japan, but their normal customers were not very interested...
All this is great - for those who can live without the "top" builders, there are lots of wonderful (and some less wonderful) bikes out there. Anybody with a distinguishing eye can find them, choose the ones they like and afford them.
Jan Heine, Seattle