The big issue with the 14-24 is flare. With the lens protruding out so far, it reaches out and grabs flare that you would not normally expect. This makes it rather specalized, and requires some careful setup and planning. The results can be worth it, but its not a lens to just walk out and start shooting, you really have to watch out for the flare. You often will not see it thru the viewfinder, only when you start editing the images and find it where it was not expected.
For all insensitive porpoises, the TS-E 24mm doesn't have any lens flare. Even straight into the Sun. (Well, okay, there's one veeeery thin faint ring. But no loss of contrast.)
I think it's safe to assume that the same engineers who figured out how to get rid of flare on the 24 will be the ones designing the 12-24, and I'd expect them to improve on their already-unbelievable performance with the 24.
And, no. This lens will not be cheap. It'll be more expensive than either the 24-70 or the 70-200, probably by 10% - 15%.
Personally, I don't expect to buy this lens. Generally, when I want wide, I want movements...I'd get the TS-E 17mm to supplement my TS-E 24mm before I got a 12-24, and I'd only do that if I felt the 24 wasn't wide enough. Considering that 24 still feels like the perfect perspective for my wide-angle stuff, I don't expect that to happen for a long time (though, if I started doing architecture or real estate or the like, I could see that changing).
I've got the 16-35, but I don't use it as much as I thought I might. For me, it's best suited for comedic stuff, like closeup shots of Shriner cars in a parade. That sort of thing is fun, but one only needs so much fun of that type....
And 12mm? I know there're people who put something that insanely wide to good use, but I'm not one of 'em.