I *do not* want a zoom, although I know the Sigma 18-35 (for example) is a fantastic lens, because I don't want the temptation of simply changing the focal length rather than changing where I'm standing, reframing the shot etc.
Understandable, but are you willing to give up IQ just to have a prime?......wow, I think that might be the first time anyone has said "give up IQ to have a prime" not "give up IQ to have a zoom......anyways, moving on
If you compare the sigma 18-35mm f1.8 to other 20mm an 24mm primes under $1000 you will find that the sigma either kicks the crap out of the prime or at least win's in convincing fashion. The only prime that seems to be a contest with is the 24mm f2.8 IS and with that you're losing 1 1/3 stops of light, gaining IS but losing speed. Heck the IQ of the 18-35 seems to be just a little better then the 30mm Art too. Just some food for thought
If you are absolutely dead set against a zoom then I'd say your money would be best served getting a used 24mm f1.4L. That might be the only prime, in that range and cost, that can compete with the 18-35