If you look at the picture above, what you see is a piece of glass in the top, connected to the camera with a long aluminium tube. That’s all, that is how simple it is made. Usually these sell for $500 - $1,000 which is quite fair for an outstanding 400mm lens. There also exists a Leica 560mm APO-Telyt-R.
The modular tele lenses (280mm/400mm/800mm) are quite heavy and quite expensive compared to the Telyt lenses, usually in the area of $15,000 - $40,000 for a lens or the set.
(via leica.overgaard.dk - Thorsten Overgaard’s Leica Pages - Page 33 - “Leica R lenses on the Leica M 240” - Leica M Type 240 Digital Rangefinder Camera - Review - Sample Photos - Electronic Viewfinder - Leica R lens catwalk - handgrips - adapters)