I ran across a very interesting article on the Scientific American website titled “Smaller, cheaper, faster: Does Moore’s law apply to solar cells?”. You can read the full article here or view a pdf version here.
In summary the author suggest that solar PV is getting cheaper at an exponential rate. I agree. The cost is getting cheaper for two main reasons. First, as the demand for solar cell goes up, manufacturers are able to lower the price of their cells by pure economics of scale. The more you make, the more efficient your manufacturing plant becomes. Solar cells are also benefiting from researchers developing more efficient cells that produce more power per square inch using less silicon. Both of these are lowering the price of the final solar module.
The author also has a graph that shows the cross over point between solar power and power produced by coal happening at about 2020.
What is graph does not show is that the US still has the federal tax credit for 30% of the cost of the system with no cap on this credit. This makes solar cheaper than coal for much of the country. If you then add in the local incentive from DTE Energy’s Solar Currents program, solar power is cheaper then coal today in Michigan.