Transit at Hong Kong with a Ghanaian passport

Hi, I have a Ghanaian Passport and I am travelling to Ghana in December, my transit is in HK and it is only an hour and half, going and coming back. how much do i have to pay for a Visa and can I get a Visa at the airport? If I have to go to the embassy, can I go for the visa now or do i have to wait till my travel date get close?