I agree that the mechanical differences are significant - engineering out the focus breathing which is a problem for focus pulls, for example.
I can add that one optical difference is that the cinema lenses are rated in T-stops, vs. F-stops. An F-stop is a simple calculation of focal length divided by iris diaphragm diameter, whereas the T-stop compensates for light loss through the lens.
I've just always guessed that engineering out the focus breathing would double the cost of most lenses as it is, as well as the focus-shift from stopping-down.
Then there's field-curvature, vignetting, and all those other lens nasties. I'm presuming that cine-lenses don't suffer from any of these problems that us mere mortals (with budgets of 4-figures or less) encounter.
Or at least, for that pricetag I'd hope that they don't have any of those problems.
And just the simple economies of scale, compare the stills-lenses of Canon (high volume, lowest price), to Zeiss (mid, mid), to Leica (lowest volume, highest price).
Situations reverse for cine, Zeiss is the high-volume low (relatively) price, Canon is the newcomer with low volumes and higher prices (for now, at least, mabe they'll drop later).
And T-Stop vs F-Stop, they're both just engravings on the lens ring, that shouldn't add much to design cost should it? Just build a prototype, stick it on a calibrated light source & meter, measure the light coming out, and get out the engraver.