I also don't think SQ tickets in economy are particularly expensive relative to others.
In low seasons, ex-Tokyo fares to Europe are very low. Less than 60,000 yen, right now (excluding airport taxes). It should be below 500 US dollars. If you fly to LHR, this will net you over 20,000 Q-miles.
This winter, I've also noticed that CX raised fares out of BKK to the U.S. but SQ has kept them low in economy. There's now a substantial gap between the two.