Air fares to Africa can be very expensive, but there are ways to save. The best way to get great airfare to the continent is fly directly to an African country from its former colonial rulers. For example, it can easily cost hundreds of euros/dollars more to fly from London to a former French colony, or conversely from Paris to a former British colony. About the only exceptions are Egypt, which has plentiful, cheap connections with the Middle East & Europe and a handful of West African destinations (the Gambia, Cape Verde, Morocco) popular with British tourists and accessible with cheap holiday flights.
Airline consolidators can also be used for discounted air fares. If you have additional travel time, check to see how your total fare quote to Africa compares with a round-the-world fare. Don't forget to add in the extra costs of additional visas, departure taxes, ground transportation, etc. for all those places outside of Africa.
See your destination's article for more specific information on flights. Bear in mind that many African countries only offer a few international flights each day, or in some cases, each week. While it isn't hard to reach South Africa or Egypt, getting to Malawi or Togo can be quite a challenge.