I actually went through a dummy booking process and booked these flights to see what it was costing...
this is not an exact science as taxes/currency change and this was done as single one-way flights.
PER - ADL (QF) - DOH (QR) - AMM - TLV (RJ) Stop -$669 (the RJ flights being the real killer here)
TLV - LHR (BA) Stop - $86
ORY - LHR - IAD (BA) Stop - $468
BOS - DOH - SIN (QR) Stop - $331
SIN - HKG - PER (CX) - $57
Total $1611
Whilst I would expect some variation on the above, my feeling is $600 variation is too much. When I booked my 280J, the taxes were significantly higher than my calculations so I ended up getting a QF supervisor to review the booking. Took a few days, but the taxes were reduced. I then made another change and the taxes went back up and I went through the process again... needless to say, I made no more changes