I have published a free Java Script that will allow you to give site visitors an instant value for their property. It is a strict math function based on assessed values. You can customize it, personalize it, and adjust it for your particular area. (And yes, it is as incorrect as you find on the big sites. LOL.)
I hate those calls that say, "I don't want you to come look, just give me a value over the phone," or when the customer says "I saw on-line that my property is worth...". This simple code lets you provide the answer for Internet surfers who have a curiosity for bad math calculations.
This calcuator is just as incorrect as other online estimators, but it will be on your site so the surfer can stay on your pages.
Happy Selling and Listing!