The RTW desk is not a travel agency and works best if you pretty much know what you want them to book for you. Starting with your question, you have a long road to travel to get to that point.

I suggest this forum is a good beginning but again you'll get a better reception if you're clearly doing some of the work yourself. Page through a month's worth of posts (not daunting, this isn't as busy a forum as some).

There used to be member-maintained spreadsheets but I don't think any are current. A rough-cut for any city can be had from ExpertFlyer - just enter the same starting and ending city (and specify any OW carrier if you want to avoid the other alliances.) A bit of effort involved but coupled with reading posts here you'll have figured out the best suspects. Hint: JNB.

Getting to the starting point and back home again later adds to the cost.

ExpertFlyer (EF) prices are always for the basic fare. Taxes and fees depend on the carrier (some impose huge "fuel surcharges", and airports along your path can extract huge fees as well.) When you home in on a starting point and know your path, use the OneWorld site to get a more accurate total including all those fees.

Besides pricing, you'll need a pretty deep understanding of a complicated set of rules to be sure a trip you've priced meets your requirements and the airlines'.

Good luck - if you don't have the time or interest in detailed planning but do want the cheapest price, find a qualified travel agent. Many might be dangerously uninformed, so be sure to choose one who's been recommended by someone you trust.