I noticed that too. Back in 2003/2004 I was regularly looking for nice Zeiss M42 lenses on Ebay in Germany and they were comparably cheap. Some prices demanded today are outright crazy. Usually lenses go for 3-4 times the price they were back then. Some even more. A 50mm Tessar could be had for 10 Euro back then, today people want 50 Euro. 75mm Biotar was about 300-400 Euro then (I even considered this price too high back in the days), today it fetches 1500 Euro. These lenses were quite nice in my view, but nothing outstandig neither in picture nor build quality. Today many of them are overpriced.
You can blame this on guys like me who put these lenses on dSLR's.
Almost all of my film era lenses are used on their respective cameras but adapters are 5-15 dollars and lenses were made better back then.
5x7 Eastman-Kodak kit / B&L 135mm Zeiss Tessar + Compur Deckel
4x5 Graphic View, made in USA! New project
RB67 Pro S /50 4.5 / 90 3.8 / 180 4.5 / WLF / prism finder / polaback
Random 35mm stuff