I see this route around $350 on multiple days ex BNA (Nashville), and I always see this on sale for $161-220 ex IAD (Washington Dulles) on the United Specials page.
You should try also the other airports in WAS also (DCA, BWI), which may give you better fares in the $270-350 range on more days.
As for posting location, as this well above 10cpm, perhaps this belongs in
"Good Deals" in Economy Class - Excellent Fares between certain city pairs - 2014?
But wow, that price disparity you found is amazing!
Safe flying.