there's also the Tokina 12-24 which has gotten some good reviews Aand is priced around $500. http://www.bhphotovideo.com/bnh/controller/home?ci=1&sb=ps&pn=1&sq=desc&InitialSearch=yes&O=SearchBar&A=search&Q=*&shs=tokina+12-24&image.x=10&image.y=7
first of all, you won't touch the equivalent focal length for less money. sigma has a 12-24 that's good - nearly as much money. tamron has an 11-18mm for under $600 you can find more info on b&h's website