I really believe it will be in the neighborhood of $1.25 Billion Coins. With so many income producing facilities, and 7 farms yielding 2x output, the value of the #8 would have to be very high to justify what it will return to the farmer.
Even if the 12x12 is $10 mill or so, that is not to much to require from anyone who already has 7 farms.
If a farmer has so most of the facilities, and all the farms, and "only" $100mill coins or so, then that farmer is not managing well. And it will be that farmer who will be first in line to object to the way the
game is being run for not letting him afford everything.
.