I think mobile apps are a great way for real estate professionals to showcase that they are up-to-date with the newest and greatest marketing techniques and makes them appear more in tuned with the marketplace etc, but don't think that many will pay too much of a premium for it (because they already feel like they have to pay for one to be on their personal websites and all of that). I could see, however, some paying anywhere from 10-20/month for it, but would be interested to see how large the market would be for it. But in my opinion 20 would be a very high number considering they could get a site for near that per month. Let us know what you decide to do, would be interested in seeing how something like this comes out.