Welcome to the wonderful world of retrofocus lenses. As difficult as it is to create any lens that focuses all of the wavelengths of light at the same point (and that gets more difficult as the lens gets wider in any case), there's more than that going on in most wide-angle lenses* for SLRs. Pentax offers a wonderful example of the transition -- they have a 40mm "pancake" lens that is about as small as a colour-corrected lens can be, and they accomplish that by restricting the maximum aperture to f/2.8 and choosing a focal length that almost exactly matches the distance from the film/sensor to the lens mounting surface.

When the focal length of the lens gets any shorter than that distance, you actually need two different "lenses" -- one that acts like, say, a 35mm lens in front of the camera, and another that acts like a longer lens between the sensor and the wide-angle lens. Both of these lens groups require more correction the wider the lens gets (light rays refracted from the periphery of the lens are bent more than rays passing through the center, and are subject to more chromatic aberration, spherical aberration, coma, etc.). That means more corrective lens elements, often more complicated focusing mechanisms to change the relationship between lens elements/groups, more interelement reflection (which means more and better coatings) -- it all gets to be pretty messy from an engineering sense. And yes, it costs more.

Take jrista's advice: the f/1.8 is more than two stops faster than what you have now, and unless you find yourself really needing the extra 2/3 stop, keep the extra $1500. If you do need to upgrade, you can get a pretty decent trade-in on your f/1.8.

*I say most because there are some lenses (particularly older fisheyes) that actually require that you lock your mirror up before you install them. You're not likely to run into them anymore, but they exist nonetheless.