The difference between a QESG and YRSG fare basis for travel in December 2014 SIN-KIX is 900 SGD return, so 450 SGD one-way. Added to this would be the change fee, on a QESG fare that is 50USD which is approx 65 SGD. So per person, at least based on these fares which may not be the fare you are travelling on, it is roughly 515 SGD to upgrade the fare to the a Y-fare.