JOHANNESBURG (Reuters) - South African financial services group MMI Holdings said on Thursday its interim headline earnings would drop as much as 30 percent following the impact of investment markets. MMI said it saw basic and diluted headline earnings per share at between 80 and 95 cents for the six months to the end of December. Headline earnings are the main measure of profitability in South Africa. Core headline earnings, which MMI regards as a better measure of its longer-term operational performance, are likely to increase by as much as 15 percent to a cap of 125 cents per share, the company said in a statement. Its shares were down 3.7 percent at 32.11 rand at 0723 GMT.