BA offered me the chance to upgrade a $750 IAD-AMS-IAD from WT+ to CW for just $9300 or something.

Personally the most expensive ticket I've known the price of was $9900 one way on Concorde, LHR-JFK in September 2002.
I've done LHR-LAX-LHR in BA F back in 2001 but my father paid for the ticket so I don't know how much it was. It is one of the more expensive BA routes though, probably more than $9900 return.