Are you talking about the upgrades they are always advertising on the GIDS if there is space?
http://www.delta.com/traveling_check...nges/index.jsp
ATL-LAS is 1747 miles, so $150
They are able to do a historical fare search. Insist on the difference between your paid F fare and the lowest available coach fare on the day you purchased (not the full coach fare, which they often try to get away with).