No. If you cancel the return leg EK will assess the cost of the flights you actually flew, ie the cost for BKK/DXB/SFO, based on fares available on the date you booked, which you say was about $5,000. They then refund the difference between that fare assessment and the fare you actually paid, less any cancellation fee (none in this case), so you would get back $2,200. It would only be done on a 50/50 basis if the fare construction is made up of two one way fares and both those fares were identical.