A safari in Kenya will run $80-100 pppd for budget, $150-200 mid, 250+ luxury using local tour operators. Tanzania a bit more. Not sure about S.A. May would be low season and you should be able to contact lodges/tented camps and inquire about packages they have and skip the tour operators.

From my limited knowledge about SA it appears that independent travel and car rental is a pretty good way to go.