I would guess the price is higher for the 24mm because the only direct competition for it is the insanely expensive Canon L version. Capitalism at its finest.
I do like my Sigma 24, though. It took some adjusting with MA to get it focusing just right. I probably would have had to ditch it if I didn't have a camera with MA. But now that I have it tweaked, I'm quite pleased, even wide open. The focus is sometimes a little slow and noisy, but I expected that based on the reviews I had read. The IQ is good on mine, and it's really amazing how closely it will focus.