Birdman: Consider 3 parts to cost: 1: Capital Cost - how much...

Birdman: Consider 3 parts to cost:
1: Capital Cost - how much money it takes to build in the first place
2: Sustaining cost - the cost of maintenance to make sure it keeps running (keeping the solar panels clean for example)
3: Operating cost - the cost of fuel (coal, uranium etc.)

So for wind and solar, while you are correct that compared to coal and gas the operating cost is much cheaper (i.e. free) Items 1 and 2 are proportionally much higher for wind and solar.

This is in part because fossil and nuclear fuels are SO much more efficient than current renewable technologies, even if a solar plant had similar maintenance cost to a coal plant for example, the coal plant will be producing far more energy, so the cost per unit of energy for coal is still smaller than the cost per energy unit from wind, even though wind is free and coal is not.