Enkosini is a great place to start looking! I spent time at CARE (Baboon Sanctuary) and Makaklali which are both offered through them.
I went to the Cheetah Breeding Project for a day tour and saw a poster up for a program for veterinary students to observe/volunteer. But, the website they provided didn't work.
Maybe the cheetah breeding project allows volunteers or you could contact them personally for more info about similar opportunities?
Here's something about cheetah conservation I found while googling:
http://www.wildlifecentre.co.za/lsp.htm
Your looking at at least 2-2.5 grand for a flight. Plus, most of the places that you volunteer at cost something (it's how they pay to keep on running, and pay for your lodging, food, etc) so depending on where you go and for how long that ranges (see Enkosini for some ideas on pricing). Then, when you get there I'm sure you'll want to sightsee, go on trips w/fellow volunteers on "off" days, etc, etc. It is expensive but it is worth every penny!
If any one has any other questions, feel free to PM me.