>Paradoxically, traveling Africa can be very expensive, because many parts are so poor and unvisited that there is no demand (and hence no supply) for reasonable transport, hotels, etc.

You just use "unreasonable transport, hotels", which is fine if you're the kind of "traveller" the author claims to be. Due to cost of living differences, they are still much cheaper than Thailand and Vietnam.

It's only "very expensive" if you go for luxury in those places (and even then, due to cost of living, it's cheaper than comparable luxury elsewhere, except if you go to some place that only caters to the elite). For exampke, if there's no transport in a country in Africa, you can usually just hire a guy to serve as a driver for several days - and it's often less than what you pay for a simple train ride betweem countries in Europe.