Looking for very cheap lenses is kind of a sport and if you look for these lenses in the second hand market of old M42 lenses then its usually easy to find something that works very well for a specific purpose.
When looking for old lenses I am usually into testing lenses for their bokeh characteristics. Its much easier to look into bokeh than it is to test a lens for sharpness or resolution. Bokeh is very subjective and therefore very difficult to parameterize.
Last week I found a lens for 15€ from the former east German ZEISS plant “Carl Zeiss Jena” – a 50 mm Tessar f2.8 with M42 mount. These lenses were manufactured for the old Pentacon SLR cameras and you can find them in second hand camera stores in Germany for below 20€ depending on condition etc (same with the old Meyer-Görlitz lenses).
The lens has 12 aperture blades and creates a very special smooth background rendering. But as usual its a “feature” if you like the circles of confusion as part of the imaging concept:
Zeiss Jena 50 mm f2.8 Tessar (M42) & PK13
The 700€ AFS VR 105 mm f2.8 renders the out of focus highlights in less appealing form, but usually bokeh with ring type of features (as in the CZJ 50 mm example from above) is often regarded as “bad” bokeh. However – for me the example from above looks just more special. Below the version from the Nikkor 105 VR:
Btw: this Zeiss lens setup doesnt focus to infinite – its only usable for macro shots.