This is a repeated question from a new user in chat. Just bcoz lvl2 and lvl3 contents are hidden until you reach the previous one. So the cost for that is also not visible in any written text available on gaming site.
So here is the calculation for the gold you required to make your homebase maximized.
Power Plant Max : 25 x 19 = 475
Military Central Max : 15 x 29 = 435
Military Office Max : 19,29,59 = 107
Command Base Max : 3 x 69 = 207
Repair Crane Max : 79+159+349 = 587
Base Extension : 49
Group Extension : 39+69+159 = 267
Flaks : 29+99 = 128
Sight Tower : 9+29+99 = 137
Jammer : 19+49+99 = 167
Detector : 29
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Total : 2588 Gold
