I completely agree to your point that a 1200 mW coal-fired power plant should be so expensive. A normal coal power plant would have cost around only $1.2 billion. However, even though expensive, a combined-cycle and non-pollutant expensive power plant is better for the future growth of this sector. It seems the SC ans USC technologies use less amount of coal per megawatt than a normal plant. This is what I got by google searching:
Supercritical & Ultra-supercritical technology
Similar term(s): SC and USC power plants.
Definition:
Conventional coal-fired power plants, which make water boil to generate steam that activates a turbine, have efficiency of about 32%.
Supercritical (SC) and ultra-supercritical (USC) power plants operate at temperatures and pressures above the critical point of water, i.e. above the temperature and pressure at which the liquid and gas phases of water coexist in equilibrium, at which point there is no difference between water gas and liquid water. This results in higher efficiencies – above 45%.
Supercritical (SC) and ultra -supercritical (USC) power plants require less coal per megawatt-hour, leading to lower emissions (including carbon dioxide and mercury), higher efficiency and lower fuel costs per megawatt.