How much you paid for the return tickets from Europe to Johannesburgh?
Which airlines are the cheapest?
I haven't checked for you but have you looked at Emirates? They are the cheapest from Athens. About 500€ return. You may have to go from Rome.
Your next problem cost wise is JNB-Maun. This is expensive. If you can stand it there is a luxury bus run which is a 1/10 the cost of flying.