I work for http://streeteasy.com . We are experts in this area - especially in real estate ;) Feel free to contact me at ch AT streeteasy.com. It is not an easy task to build what you are looking for. We've been building and improving our system for two years now (ruby on rails.) Scraping the data is just one part of the problem. Validating the data is also a big issue. These sites often have incorrect or stale information. MLS's are good, but they may have restrictions on what you can do with the data, or (as in NYC) they may not even exist.