Hi Casey,
Great question! Technically, solar energy is actually free, of course, but the equipment to harness it isn't. I suspect you are asking about the photovoltaic systems (PV) that converts sunlight to electricity, and this is actually a question lots of people are asking right now! To answer, we need to think about what is happening in energy. The fuels that we have been using for the last 100 years or so are becoming more expensive to get - mostly coal through mining, oil and natural gas through drilling. The stores that were easy to get to are gone, and we are left with the stores that are deeper, lower grade, and more environmental costly. As the cost of "convential" fuels increase, solar and wind look better and better. That is one side driving this issue.
The other side is what Mitzi described. The PV equipment costs have dramatically decreased recently, mostly in PV modules and inverters. Only 2 years ago we said the range for an installed residential (home) PV system would cost $7 - $10 per watt. Today we see that more in the $5 - $7 per watt. The cost of the modules themselves has decreased even more, with wholesale prices dropping from $4.50 per watt three years ago to under a $1 a watt for large orders now! Most experts agree that PV module prices have bottomed out, but inverters (the next major piece of equipment) set to have deep cost reductions in the next 2 years.
Additionally, as more systems are installed, we are learning how to do it faster, smarter, and with less materials. These installation costs, which are becoming more of the cost of the systems, continue to decrease.
So as convential fuel prices go up and PV equipment and installation prices go down, solar becomes more and more affordable.
Thanks for the questions,
Kelly