When looking at an itinerary you need to include all segments in order to be sure you're not exceeding the per-continent flight limits. In your case, a valid xONEx routing would be something like

JNB-HKG-YVR-DFW-MBJ-MIA-BOS-LHR-DME-LED-HEL-LHR-JNB or JNB-HKG-YVR-JFK-MIA-MBJ-MIA-BOS-LHR-DME-LED-HEL-LHR-JNB or some such.

BOS-DME requires a connection through somewhere in Europe, presumably LHR, so that would make the European part of your "original" itinerary something like ...BOS-xLHR-DME-LED-LHR-HEL-LHR-JNB, which is one too many segments in Europe.

Current pricing according to Expert Flyer is ZAR 21,800 for an LONE4 and ZAR 48,000 for a DONE4. Those are BASE FARES, and taxes and fuel surcharges can add 10% - 20% or more to those prices depending on carrier, stopovers (e.g. the UK) and other factors.

Note your route above only contains 12 or 13 segments, so you could include the remaining segments in your ticket fairly easily, e.g. HKG-DPS-HKG for Bali, or possibly originate in Namibia rather than South Africa and make your first segment WDH-JNB. Prices originating in Namibia are almost identical to those originating in South Africa. Any xONEx begun in Africa can be ended anywhere else in Africa, so if you wanted, say, to conclude in Tanzania or Kenya, you could do so.