I know this is old news, but a reader pointed out to me (thanks Andreas) that the Leica APO-TELYT-R 1:5.6/1600mm may not be the most expensive lens in the world. Take a look at this monster - Zeiss Apo Sonnar T 4/1700mm:
The "tiny" black cube at the end of the lens is a Hasselblad camera.
I could not find any info on the price, but this is definitely the largest telephoto lens in the world (beats Leica by 100mm).
Google translation (Source):
Such a photo optics, the world has never seen! Developed for animal photography from a great distance in the wild increases of this super-telephoto 21-fold. With the aim to achieve the highest quality opted for the client and 6x6 medium format camera as the best in his experience the Hasselblad 203 FE.
With 1700 mm focal length and intensity 1:4 put this lens unprecedented demands on optical glasses, assembly and manufacturing technology. With 256 kg in weight are all well known tripods totally overwhelmed. So has Carl Zeiss brand new handling and Fotografiertechniken redeveloped.
With his experience in satellite and space optics and micro lithography lenses for the semiconductor manufacturing Carl Zeiss has the challenge of STL likes adopted.