Why are fast wide lenses so expensive vs fast long lenses? I don't understand it...an 85mm f/1.8 has to capture more light on a reduced angle (hence the large front aperture) but a 24mm for example has a wider viewing angle to capture more light! Why doesn't Canon for example have cheap wider angle glass? I just dropped $290 for a brand new Canon EF 35mm f/2 but why is the 35mm 1.4 more expensive that some of the longer lenses just as fast?! Doesn't make sense to me...its also a pain trying to get wide shots on a crop sensor body...I mean yeah I can get wide shots but not wide shots at f/2!
50mm lenses and moderately longer can be made with a modified double gauss design, which is a simple symmetrical design that cancels out aberrations well at the cost of soft corners. But it's very simple and with good IQ and you don't need special glass.
I believe the 35mm f2 has a simple, relatively symmetrical, design and it's not a large lens. But most fast lenses wider than 50mm would bump up against the mirror in a dSLR if made with a symmetrical design (focal length is distance from optical center to the film plane when focused at infinity and I believe near-symmetrical designs put the optical center very near or at the physical center of the lens). Symmetrical designs lack a lot of aberrations and have good contrast because they are simple, but the corners can get soft. But when a lens gets too fast (the 50mm t0.7 lenses Kubrick used couldn't be used in a reflex film camera) or too wide (14mm f2.8) or a mixture of both (35mm f1.4) a special design must be employed so that the lens is longer between its optical center and the film plane than its stated focal length (otherwise the mirror would hit the rear element when focused at or near infinity!).
Telephoto lenses have a special optical group near the film plane that simulates a lens that is longer than the physical length of the lens (distance from optical center to film plane). Meanwhile, retrofocus (reverse telephoto lenses) have a similar element in front of the lens, and this is the design that is used for the 35mm f1.4, for instance. The downside of retrofocus lenses is that they have lots of aberrations relative to symmetrical designs: more distortion, CA, etc. But...by using very complex designs and rare glass (low dispersion or whatnot, aspherical elements, etc.) you can fix these aberrations. There are two problems: lots of large lens elements made of weird glass is incredibly expensive and difficult to design and you get so many air to glass surfaces that contrast is low. With advanced coatings and computer designs, the only real issue is cost. It's worth noting that Zeiss and Canon have 35mm f1.4 lenses that outperform their 50mm f1.4 lenses and Leica (rangefinders do not need retrofocus designs) makes a retrofocus 35mm Summilux. Zeiss is making a retrofocus 50mm f1.4 that will be thousands of dollars. So the IQ is better, too, in theory.
It's worth mentioning that Canon has a 22mm f2 for its mirrorless and Fuji has an 18mm f2 for its rangefinder... So when we all go mirrorless lens options should open up a bit!
If you need to go wide and fast for cheap, just buy a 5D (or 6D?) and a 24mm f1.4. There are great PL mount options (Master Primes, S4s, Superspeeds are my favorite and the least expensive...) but they're ridiculously expensive except to rent and they're all PL mount. Sadly, there's no market (in stills) for a 18mm f1.4 that's APS-C only and it's too hard to make that lens for FF. And the video market is still much smaller than stills and most videographers are tremendously ignorant. I'm hoping Samyang will make that lens (their cinema lenses are excellent except for breathing, but missing this focal length and 50mm f1.4), but there's so much misunderstanding about lenses and formats among young videographers and cinematographers (it's embarrassing that these people charge money) that few would appreciate how important it would be! Until then...the 17-55mm f2.8 IS is nice. Even the kit lens is nice at 18mm f3.5--better bokeh, too.
It's much more complicated than this, btw. Even if retrofocus vs symmetrical weren't an issue, coverage would be. And even symmetrical designs aren't truly fully symmetrical. Check out large format ultrawides (which have horrible vignetting) and see how expensive they are and how curved their front elements must be. And the vignetting is terrible! So even symmetrical ultrawides are tricky. I wouldn't chime in (I know nothing about lens design) except that a lot of the answers here are wrong or gross simplifications worse than my own.