I would encourage you to do some research, before you commit to anything. Buy a guidebook, read some magazine articles, and come up with a plan. What do you want to see? People or animals? City or rural? North or South? As JDiver says, 'Africa' is a big place with a lot of countries, each with their own charms and trials, each with something to see.

It's certainly possible to get by on less than $50/ day, but you'll have to choose your destination carefully (and try not to travel the length of the continent), live frugally and take buses. You can pretty much cross out most of the big national parks and game reserves on that budget, although I'm sure that smaller ones will be available to you.

Where do you plan to stay, and how much luxury do you need? A friend cycled through Mozambique spending most nights in a bivvy bag by the side of the road, and occasionally taking up offers of local accommodation. Very cheap, but you need to know what you're letting yourself into.

If you want clean, comfortable, US-style accommodation you'll generally have to pay for it. Like B747-473B says, $500 for a decent room in a city is not unknown.

One place which, in my experience, you can scrub from your list if you're on a budget is Botswana. Their strategy has been to attract high-value tourists who will pay hundreds of dollars a night for camping, and several hundred more for a private plane transfer. You can hang around in Gaborone or Francistown for a couple of days on a budget, but there is little point in doing that.

It all comes down to how comfortable you are 'going native'. I like to think that I'm pretty good at it, but when the chips are down the credit card comes out...

Either way, it is important to reach an agreement with your friend before you leave. If he is planning on $50/ day and you are happy to spend $200, it might be an unpleasant trip for both of you.