There is a website dedicated to the camera industry of Dresden, Germany, including the Praktiflex
Although the author states he is reluctant to answer commercial questions, you can always inquire about the value of your lens. The site lists prices of Practiflex lens combos, the Biotar fitted cameras being the most expensive in 1939.
The website is in German, if you need any help with composing a message in german just let me know.
BTW I don't think 80 pounds for a single copy of a M40 to MFT adapter is expensive (there are even people who use teflon tape to screw M40 lenses in a M42 mount)