As many people have stated, there is not a substantial or obvious performance difference between professional grade lenses from most manufacturers. They are often using very similar lens designs (unless they are very exotic).
So, why is Zeiss more expensive?
1) Zeiss does indeed have very high build quality and QC. I've not experienced a dud lens from Zeiss (but most of my experience has been with the Hasselblad line).
2) Zeiss originally developed and manufactured many of the 'classic' lens designs which other companies copied (such as the 4-element Tessar and the 6-element Planar). So if you are obsessed with having 'the original' of a design, they are often the source.
3) German-made lenses are almost always more expensive than lenses manufactured in other countries(see prices on Schneider and Leica lenses as well). Even German NAMED lenses are more expensive (See: some contax zeiss lenses, rollei made-in-singapore lenses, leica made-in-canada lenses, all cosina-zeiss and cosnia-voigtlander lenses).
4) In side-by-side tests, Zeiss very often (not always) bests all other brands for optimal resolution. However, if you are outside of a lab, it is debatable how much this matters. If you want to be absolutely sure you are getting every drop of resolution possible it is occasionally advisable to purchase a Zeiss lens. This is not true of all focal lengths or makes of course, and many vary sample to sample (although, again, Zeiss has a reputation for good QC and less sample variation).
TL

R - Fancy-name brand lenses are like fancy name-brand anything (cars, audio equipment, tools, etc.): possibly technically better, but the difference probably doesn't matter in use.