Thanks to all for your replies. Og thanks for the suggestion to look up the 'best flights' web site, quite informative. AIB, my routing is as follows: MEL-LAX-DFW-BJX (stop & own way) GDL-DFW-GUA (stop & own way) CUN-MIA-MGA (stop)-MIA-LPB (own way) CJC-SCL-UIO (stop)-EWR-LAX-MEL. Not quite the round the world trip as stated.
At no stage am I stopping in the USA and the departure taxes for Nicaragua and Ecuador have not been included in the taxes already paid up front. As mentioned in my original post, Qantas did provide a breakdown of the taxes but the shock at the amount (almost double from late last year) led me to raise it here. Once again thanks to all for your help. Cheers.