The DONE5 ex-South America is $8400 but if you don't want lots of stopovers/weird routings BA are marketing an I class fare ex-LIM/UIO/BOG/CCS for $5999 which basically is a SA-SYD/MEL return with unlimited stopovers as long as you stick to the specified routings:

UIO(MX)MEX-LON(BA/QF)SYD
UIO(MX)MEX-LON-SYD
UIO(MX)MEX-LON(BA/QF)SIN/BKK(BA/QF)SYD
UIO(MX)MEX-LON(BA/QF)MEL(QF/BA)SYD
UIO(BA/AA/LP/XL/VH)MIA/DFW-LON(BA/QF)SYD
UIO(LP/XL/CO/AV)HOU/NYC-LON(BA/QF)SYD
UIO(BA/AA/LP/XL/VH)MIA/DFW-LON-SYD
UIO(LP/XL/CO/AV)HOU/NYC-LON-SYD
UIO(BA/AA/LP/XL/VH)MIA/DFW-LON(BA/QF)SIN/BKK(BA/QF)SYD
UIO(LP/XL/CO/AV)HOU/NYC-LON(BA/QF)SIN/BKK(BA/QF)SYD
UIO(BA/AA/LP/XL/VH)MIA/DFW-LON(BA/QF)MEL(QF/BA)SYD
UIO(LP/XL/CO/AV)HOU/NYC-LON(BA/QF)MEL(QF/BA)SYD

To max it out you could do:

UIO-MEX/MIA/JFK-LHR-SIN/BKK-SYD/MEL-SIN/BKK-LHR-MEX/MIA/JFK-UIO

Fare basis is ILRCLAC2. I was able to price it out on ITA and find reasonable availability in I class on a variety of dates although week-ends could be problematic (perhaps no more problematic than D class?)

Note that Ecuador and Colombia (perhaps Peru/venezuela as well) levy taxes of 5-?15% on tickets originating in their countries that add up to $1000 in taxes but these will also be leveled on the xONEx as well and if you are able to combine a cheap fare from somewhere else om the same ticket eg MIA-UIO so that your origin is not one of these countries you should be able to avoid this added tax