You must have a 10-22mm for real estate, specifically inside shots. Outside shots I find can go either way, the basic 18-55mm that comes with the camera works good and I also find use for the 10-22mm when I am taking a shot of a house with a small back yard, the lens works wonders to "fix" that.
I have a Canon Digital SLR ($600ish) and a Canon 10-22mm wide angle lens which costs about $750 as well as a nice flash that was $450+.... I read reviews for months with regards to Sigma vs Canon and Canon was the winner in my book, more expensive? probably but I meet so many photographers who use it for real estate. I am a real estate broker and NOT a professional photographer but I now have all the equipment the local pro photographers have and I will tell you, the 10-22mm is a must if you are taking photos for a real estate agent. Sure I could have spent $5,000 and not $2,000 for my equipment but I just need very good photos and can't spend more than 25 min taking shots and 20 min to get them into my listing, don't have the time that most photographers have or need to perfect there photos. I do not do HDR or any other methods that take up too much of my time, my time is focused on selling real estate not taking photos.
BTW the only reason I even bought all my equipment was because my photographer went on vacation for 2 weeks and I did not realize how desperately I depended on him and had 5 houses that needed photos that week so I can't be put in a spot like that so I bought all the equipment for more of a back up when he is not available but I now do 90%+ of my own shoots since I love to do it and I can get it done in 20 mins, go home and get my listing on the mls all within an hour vs waiting 1-2 days for my photographer to send me the photos. I can always go back to take more shots if I do not like what I see but that has never happened yet.