There were thousands of 1960-1980 Japanese bikes of all makes and sizes sold in North America and around the world - most of them 50-250cc displacement - still around being ridden (or parked somewhere) up into the 80's. Look at a late 70's or early 80's newspaper want ad section and you will see many various displacement used bikes for sale at respectable prices.
Then modern sportbikes came out so that these bikes were unfashionable - and prices crashed. Young riders wanted a cool sexy new 600: not a ridiculously puny old 50, boring puny old CB350, smoky puny old RD400, or slow/overweight old CB750. The low prices attracted buyers who exported them back to Asia by the boatload. Smaller displacement bikes rule in Asia: for example in Japan (until recently, I think) getting a drivers license for over 400cc bikes was extremely difficult. The Japanese CB400F was 398cc instead of the export version's 408cc because of this. "Big" bikes are quite rare throughout Asia. Motorcycles are basic transportation there - not fun toys. You could buy as many 150-400cc bikes as you could find, pack them in containers and ship them there, clean them up, and still sell them with a good profit.
The larger used bikes were not wanted overseas as much and many stayed here to be cherished, slowly rust in barns, or be ridden to exhaustion and scrapped. In North America it's much easier to find a good CB750 than to find any 70's 250 except rusted out junkers at scrapyards.
I believe the inflation in interest and pricing for the SOHC4 bikes (and just about any other "classic" bike) is from older people who stopped riding because they had a young family and now want to relive their youth since the kids moved out. I don't see many young people on classic bikes. Swap meets are mostly old guys telling stories about the "good ol' day"s and selling each other parts for their toys. Vintage bike clubs are almost entirel comprised of old men who argue the same dumb arguments they had in the 70's about which bikes are good and which are crap.
If you want to get back in the saddle, a Harley is just too much damned money for a 1940's bike regardless of whether it's an actual 40's model or any other year up to their new 2014 model 40's bikes. English bikes are also overpriced and have serious reliability issues (the reason Japanese bikes killed Brit bikes in the 70s). Only a masochist would buy a used Italian motorcycle. So, if you can't justify paying for the HD nameplate or for any decent new bike, a 60's to 80's Japanese model is ideal. Reliable, reasonably quick, parts are (were?) still available fairly easily, and the cost is really pretty decent excepting some special cases (the "sandcast" effect).
