I've never bought Avgas in Africa, but I have bought some in Canada, at a remote outpost, whose cost was $6.83 per liter, and you have to buy the whole drum, use it or not. I was happy to pay it, and even more happy that the whole drum fit in my plane.
In northern Finland two years ago, I paid 3.85 Euro per liter, and that was at a large airport.
I respect the fact that some "off the beaten track" trips, can be very costly. Pilots must go into that aware.