Doesn't the base fare match accross the carriers, and the plating carrier therefore irrelevant unless you change codeshares/operating carriers withing the same fare. A quick glance exOsl DONE5 shows BA's fare ~30USD higher than QF/QR/AA etc. My understanging is that you cannot really play an aribitrage with plating carriers unless they have different base fares filed like it sometimes does happen. Otherwise to minimize taxes and surcharges, the only way is to look at codeshares. Gvernment taxes are set, you can only minimize those by changing your route/stopovers.