I may be part of Generation Y, which could be held responsible for this epidemic, but I'm sure there are still lots of enthusiasts that can relate to what I'm talking about. The prices of old Japanese bikes are skyrocketing! When I first found my passion for these bikes, it was affordable enough for an 18 year old like me to pick up an old Honda, break out the wrenches, and be on the road for less than 1000 bucks. Things have definitely changed in the last five years.
The CB450 has become a common platform for Cafe conversions in recent years
I hate to sound so negative; it's the simple model of supply and demand in effect. These RD, KZ, CA, CB, GS, and XS models are becoming incredibly hip amongst young riders. They are highly customizable, relatively reliable, and most deliver good mileage. My 1970 CB450 with Wiseco pistons delivers 45 miles per gallon no matter how I ride. My point is that what used to make these bikes so cool was their entry-level prices and modest performance. Nowadays, as more and more of these old standards are turned into café racers, less and less NOS (new old stock) parts are on shelves. These bikes have stepped into the "vintage" or "collectible" category of bikes, and the days of thinking they don't have value in today's market are gone.
Of course some Grandpas are still out there, letting their old bike collect dust and unaware that they can call this a "barn find" and sell it for thousands of dollars. These bikes usually don't end up on craigslist or eBay though, you have to go out and find them where they sit. My riding buddy Cameron just picked up a running CB450 Black Bomber for 400 bucks. That is quite a rare purchase in today's market. You can't even get a good tank for those for less than 400! The days of cheap vintage bikes are over. The 21st century is fully upon us. Expect to pay a premium for these old machines and the nostalgia that comes with them.