It will varry but expect to pay at least $300-$700 for the ticket. A lot times flights for your return trip will cost more because you will be forced to land in a different place when flying out of Africa to check the flight. Direct flights are rare to all but the most stable countries.
It may depend on the airline, airport, and season. I checked with Continental Airlines, and a June trip to Ghana from JFK International Airport would cost about $3000.