Considering the fact that a 500MW coal-fired plant can cost upwards of $1 billion just for land acquisition and construction (not considering operational and maintenance costs or fuel costs), a wind farm with equivalent energy output, zero emissions and no-cost fuel (wind) comes in at about $600 million.
'Equivalent energy output' is incorrect. The original author is confusing 'installed capacity' - the power it could generate under ideal conditions - with output. This is a very important distinction.
For example, if you have 500MW of coal capacity and 500 MW of wind capacity, as long as you have optimal fuel for each system, they will output the same amount. In the case of coal, the operator can control the fuel supply (mostly) and run at any percentage of peak capacity for as long as is desired, up to 100% of the time. For wind, the operator is at the mercy of how much and how fast the wind blows and can only operate at peak capacity when conditions are perfect, which is less than 15% of the time. The concept is called 'capacity factor'.