I'll write more about this in an upcoming blog post, but we've decided that we'll make power plants progressively more expensive. Every power plant beyond the third will be more expensive to build than the previous one. We're still testing how quickly the power cost should rise. At the moment, we're looking at base costs of 90 power for your 1st through 3rd power plant, 100 for your 4th, 120 for your 5th, and then on to 150, 190 , 240, etc.
--Willem--