Price history chart of crystalline silicon solar cells in US$ per watt since 1977.
Own work, based on Hanjin's 2013-version (en español), amended with average sales prices for 2014 and 2015.
Original source data 

1977–2013: Bloomberg, New Energy Finance, (archived)
2014: based on average sales price of $0.36/watt on 26 June 2014 from EnergyTrend.com
2015: based on average sales price of $0.30/W on 29 April 2015 from EnergyTrend.com
compare to current spot-market prices here.
Rfassbind