Except, your equation completely forgot the additional production value for the remainder of the reset after you built it up. Nor the costs for building said land, buying tech to maintain tech levels, and buying more units to defend your new land.

You are only thinking in terms of NW gain after 2 turns.

I would gladly attack someone with 1000 troops per acre, let alone the 13 you calculated if there are at least 12-15 days left in the reset. That's only a 5-7m jet PS on a 10k acre country. I might lose 900k jets in that attack, but that 800 acres gained will more than outproduce what is lost in the long run.