Solar PV system prices are declining fast

Average U.S. PV system prices over time. - NREL, 2015

Photovoltaic system pricing trends were analysed in the presentation of the National Renewable Energy Laboratory (see above). In 2007 - 2014, prices for residential and commercial PV systems came down by 55 - 65%.
The report from the Solar Electric Power Association states that in U.S. average residential solar prices in 2014-2015 clustered around $3-$4 per watt, while prices for nonresidential and utility-scale projects came in lower at $2-$3 per watt.
According to GTM Research’s latest report, average global PV system installed costs will fall from $2.16 per watt in 2014 to $1.24 per watt by 2020, a 40 percent decline, with variations by region and market segment.