have ever in your planted a tree it actually cost 3-10 dollars per plant I have 12 plants in my house I know how much it cost to plant & maintain them. And if u do in extreme amount u can some money per plant but still at the very least it will cost 1/2 dollar. Plus this project is not being done by free volunteers but by paid employees so it will actually cost around a 1 dollar per tree plus artificially planted trees usually dies in few weeks so u have to maintain them for nearly a month after planting them so it will add even more cost.

Come there is a reason why countries don't do projects like these and the reason is cost.

Then there is another factor bush fires ,too much biodiversity but I will not go into those.

That's why governments try to plant millions of trees not billion.