I have a Sigma 10-20 and it certainly is the biz for opening up a view of a room. Every estate agent in the country should have one! If you are looking for a cheaper alternative there are lens / filter add on that screw onto the thread of your kit lens. Here is one on eBay for £15
http://www.ebay.co.uk/itm/58mm-0-45X...item20c4c19348
They are not great for high quality images but will give you good enough pics for a basic before and after of the inside of a cottage if that is all you are after.