JNB to DAR is expensive - you might be better off finding a cheap flight to Kenya (check charter airlines from Europe) - or a regional flight to NBO or DAR from Egypt/Middle East (Air Arabia?). The options will be limited if using a buddy pass. You wont find many low cost alternatives as Dar isn't really served by any.

Check Ethiopian Airlines using a travel agent (e.g. cheaptickets.com).

Why ZNZ? It is a great place to visit but you might be better of elsewhere if cost is an issue - Kenya is especially cheap at the moment/can be combined with ZNZ - and (mostly) safe.