Bernd M: I don't understand why I'll have to pay US$ 5.700 for a Zeiss 135mm F2.1 when I could buy a Zeiss Sonnar T 135mm F1.8 for US$ 1.800. What makes the Cine Lens so much more expensiv? Can anybody tell me?
As others have said, the manufacture, color accuracy and durability are all important.
But probably a little bit less so as the world has moved to digital. They may be looking at either a shrinking high end pro market, or a rising lower end market, where a few thousand bucks might actually make a difference.
The issues with color balance and T-stop were much more important in the film era when a screwed up shot would be very expensive. Digital is better that way, seeing what's "in the can" in real time. But subtle color corrections are still a hassle and costly of data processing, but not deal breakers.