None of the examples you quoted above that you tried through the online tool included xLHR which will add on significant taxes/fees (an additional 250,000-300,000 KRW) no matter what airline tickets.
Actually $543 USD (729,820.00 KRW) seems extremely reasonable for a DONE3. I paid $496.60 USD for taxes and fees for my ACIR26 with 4 stopovers last fall. Needless to say it was different stopopvers, but obviously it did not include the very expensive LHR stopover.
Last edited by aaupgrade; Mar 27, 2009 at 4:00 pm
Reason: Added comment about total cost