backpacking budget for africa???

Hi everyone! I am planning on continuing my traveling with a trip around africa and am planning on being there for about a year and i have been trying to find out how much it will cost me. So if anyone has done a similar trip (and done it cheaply) I am really interested in how much they spent! Also if anyone has done some volunteer work over there i would love to hear about some good organizations. Thanks!
_kels