Early this year(in March), I have travelled SFO<-> LHR on BA non-stop flights, only paid $800 r/t all-in for two people, plus two nights free hotel stays.
But during the high season, such as June, July etc., you expect to pay over $1000/person for r/t.
No matter where you are, the only way you need to do is to go online and then find the best fares.
There is a travel website that people post cheap airfares
at very moment:
http://www.flyertalk.com/forum/forumdisplay.php?f=1
i.e. two weeks ago, UA had a sale on expedia from LAX to east coast cities like IAD, JFK etc., the fare was only a little over $100 r/t all-in.
Northwest has a fare sale right now from Bejing, Shanghai or Guangzhou to US only RMB3840 r/t.