I have this lens and mostly use it at f/2.0 for shooting people, where it exels. It is reasonably sharp in the center wide open, but also somewhat "dreamy" overall at full aperture. I have discussed this lens with Sherry Krauter, whom I believe was working/training at the Leica factory in the early 1960s, and she stressed that it is not the same as the Summarit and everyone at Leica understood that it was an improved design.
I do not think there would be a reason to pay a large premium for a prototype of any lens and then use it. It might well make sense for a collector to pay a premium. You could buy several exellent 50s for $2400. If I were looking for a first 50mm to use primarily for travel and street photography, I would give a lot of consideration to the Summicron v3 (1969) and the Zeiss 50mm/2.0 Planar ZM.
I see there is a "prototype" listed on eBay with a regular serial number and early barrel design (knurling on the high parts of the focus ring). I doubt that qualifies as a prototype, and if it does then I have an earlier prototype (the 20th serial) and apparently should be shopping it around.