Sites that show the difference between price and estimated value

[Phil Sharp]

There are a lot of sites that show some type of estimated value for a home, but I haven't been able to find any that compare list price to that estimated value.  Or that let you search for house with, say, $50k or 20% difference between list price and estimated value.  Are there any sites that readily show you the difference between price and value, and let you search/filter for this differential?