Hello, I can develop a Google Chrome extension for you with the functionality to get prices from real estate websites, compare the price with some calculated or obtained by means of ajax request param and modify the content of a given webpage.
I am a Javascript specialist, I have developd 3-4 chrome extensions before, but all of them were private extensions, never published, so I cannot provide link to show you any of them. However, your project description outlines very common functionality for any website: get some data, perform calculations, display the result. All of that can be done by means of content scripts with Google Chrome.
So, if interested, feel free to contact me. Regards, Alexander