LJT,
As an exercise log onto any airline's website and book both return and one way tickets from anywhere to anywhere and you will find that a one-way ticket is ALWAYS a lot more expensive than a return.
I am not sure why it's like that but it is not just BA picking on Nigerians, believe me. My boss paid almost double on a one-way LOS-JNB for me when I had to get home for an emergency.