There is no way to know excatly how much traffic an expired domain receives, without actually regging it and puttting it on parking or something.. As Dynn advised you to, Check all the stats of the page (IE Backlinks, Page rank, alexa rating) to get a picture of how much traffic the domain was receiving when active.. Or... (Shameless promotional plug imminent!!!!!) You could just swing by my site and buy one of mine that is allready checked for traffic