One of the cheaper ways to see some beasties would be to hire a self-drive, rent or take your own camping gear and camp in the various government rest camps in Etosha and other Namibia national parks. There are still considerations - food, park fees, camping fees, etc. etc.

We've spent some economical times (but again, transport costs must be taken into consideration,) in Malawi as well.

$50 a day? Not most people. And "three weeks - JNB - include Tanzania" I'd say please tell your friend to do a bit of study - Africa is over 50 countries, and the second largest Continent (scale: Africa is three times larger than the USA). The more you move around, the more it will cost.