This paper from the Institute for Security Studies claims that "[Executive Outcomes'] two-and-a-half-year contract with the Angolan government earned the company over US $40 million per year and several lucrative oil and diamond concessions." It goes on to value the company's diamond concessions in Angola and Sierra Leone at "over US $3 billion".

Given the complexity of EO's corporate structure and relationships (particularly with large mining companies active in the region), and the secrecy in which it shrouded itself (PMCs are generally not known for their corporate transparency), getting to a clear dollar figure for its total earnings from Angola (or, in fact, its earnings at all) is next to impossible.

Edit: typo