I agree that the prices of R20s are increasing. But in reality, so are all bikes-especially the ones that have received the "cool" stamp of young hipster approval. I myself don't follow the crowd and never did. I ride the bikes I choose just as I did when I was younger. I fit the bike to my own body and use. Then I use, maintain, and keep them practically forever. Or until the bikes bury me.
But to keep some comparison in prices, my own Raleigh cost far less than my Brompton did-five years ago. I think that 200 US dollars is still quite cheap.
I had one of those retro u-folders in my stable last year (a 1980's Peugeot) but I sold it almost as quickly as I acquired it. It was in fantastic condition--the owner had barely ridden it. However, while stylish, it was heavy little thing, very awkward to fold, the side-pull brakes were completely non-optimal at gripping the rims (and the LBS specializing in vintage bikes said there was nothing they could do to make them work better), and being limited to a 3-speed hub was, well, limiting. I could see it being a fun bike to ride if all I ever had to do was make 1-3 mile jaunts on flat streets, but since my commutes are generally 10 miles or greater (and with hills), that horse was not right for my course. Today's folding bikes are so much better engineered in terms of folding and road performance, I think the only thing these retro folders have to recommend them are vintage charm and bomb-proofness (steel frames and 3-speed IGHs mean they'll be around for a long time).