Online resources for researching comps

[Khari F.]

What are some good online resources to research recent purchase price of properties for comp? Is there a tool that can compare difference in sales prices of properties within neighborhood based on their amenities or lack of? I figure that would be best way to calibrate costs of value-add - i.e if prop w these features sold for x amount more than prop without with these features > how can I add those features for a cost less than the difference?