Assuming the $100 a day does not include transportation, this is pretty trivial in most of the world.

For example, I was just in Peru. I stayed at a nice mid-range place in Cusco for $20 a day. Figure another $10 a day for meals. Bolivia or Colombia would be even cheaper, I believe. Even in Chile, which is relatively pricy for South America, hostels are widely available and cheap.

Last year in Cambodia, I never paid more than $15 a day for accomodations - and that was for a fairly high standard. Again, cheap hotel rooms and cheaper food abound in Southeast Asia.

By the way, for those who are suggesting overland trips in Africa, you can do South Africa on your own far cheaper. The tricky thing for seeing wildlife is park fees, which can be significant - often 3 times the cost of transportation.