I added * 4 because you bulit 4 at once. However this was wrong. But look what the price is fro strong wall then 4000 and for steel wall 20 000. (just for 40 pieces which may enough to cover one side)
upgrade while replacing?
I mean this
Even if you mean 1000 = 4 blocks its just 20 of them. 20 blocks for 5000 $ steel ?