Cocoa farmers usually get only about 25 cents for each pound of cocoa beans they sell. This is not even enough to pay for the cost of growing cocoa trees.

(In the last sentence, what does 'this' refer to?)

In the second sentence, "this" is referring to "about 25 cents for each pound..." -- in other words, to the amount of money they get per pound of cocoa beans.

"This" refers to "25 cents per pound of cocoa beans they sell."

In the last sentence, the word "this" refers to the fact that cocoa farmers usually get only about 25 cents for each pound of cocoa beans they sell. The sentence is highlighting the low income of cocoa farmers, which is not enough to cover the cost of growing cocoa trees.