OK, the study simply argues that by adding 10 million squared kilometers of forests – the area equal to the territory of the U.S. (or Canada, if you wish), 2/3 of the man-made emissions of CO2 since 1800 could be removed from the atmosphere (they apparently pre-decided that the elimination of 2/3 of a problem is a good plan). Planting a tree costs $0.30, they claim, and because one needs about one extra trillion of new trees (the number of trees would be increased by 1/3 in this project – they did use the same estimate of the number of trees in the world as I mentioned above), the total cost would be $300 billion. This is the overall amount, not an annual one.So for $300B over 30 years we could eliminate the excess carbon dioxide that we've put into the atmosphere. Even if this figure is off by an order of magnitude, this is a huge win compared with the $122 Trillion that the UN wants to "fight Climate Change".
Alas, planting forests does not allow the industrial scale graft that the UN has come to expect.