Lately I've seen something like $150 off each way or something like that. In the old days, I used to fly SEA-LHR on BA and credit to BA, and each return would get me almost enough miles to MFU the eastbound of the next trip--basically every trip was in CW on the eastbound and WT+ on the return. I've actually bought a return SEA-LHR for less than $1,000, but that was 3 or so years ago.