Flying to Gabon will cost you about $2,400 return and $2,000 to Ghana.
I don't rate Cameroon or Gabon beaches. I don't think Ghana's beaches are much better.
I suggest you go to Kenya - the cost would be $1,600 return airfare to fly.
I suggest this because Kenya also has coral reefs (like at Watamu) and this makes snorkeling or diving interesting. The Ghana, Cameroon and Gabon beaches don't have coral reefs off them.
At Watamu you will find turtles, whale sharks and fish of every colour. Not so in Ghana or Gabon or Cameroon.
In Kenya you could also take a two day safari as part of the beach holiday and see elephants, lions etc.
Kenya has waaaaay better wildlife than the other three countries.
The beaches in Kenya are superior to the other three countries n every way - try Turtle Bay beach, or Watamu beach, Diani Beach, or Chale Island beach.
Lived and worked in various African countries. Been in Africa for most of my life.