There are a number of fast X-Ray lenses out there. Rayxr De Oude Delft 50mm f/0.75, Rodenstock XR-Heligon 50mm f/0.75, Precise Optics 50mm f/0.75, Varo Optical 50mm f/0.75, Leitz 65mm f./0.75. There are also a number of longer f/0.95 – f/1 lenses in the 90mm range from the same lens manufacturers.

There's a company Miyazaki R&D (well, it's just one man hand-modifying the lenses, not really a company) that has successfully converted the De Oude Delft 50mm f/0.75 to Leica M-Mount so that it can be used "normally: ie. not macro only, but all of these lenses are still fixed focal length, fixed focus, and fixed aperture. They will only be useful for mirrorless cameras with short registrations and best with a camera that has live view. Also be aware that the lens conversion isn't cheap.

These lenses have seen a jump in price from about $200-$1000 depending on the condition and maker. Rodenstock and Leitz typically command a higher price because they are well known in the optics game. We all know that Leica/Leitz lenses have been the 35mm film standard forever and Rodenstock made the best large format camera and enlarger lenses.

I hope this clarifies most of the questions about these super-fast X-Ray lenses.