Here'a a graph with some quick data grabbed off the internet:
http://i.imgur.com/qbhSNbH.png
The fare was $0.30 in 1970. From there, I applied the inflation rate to compare with the fare prices. The price of a ride should be reduced on this graph once unlimited metrocards were introduced because those are cheaper and account for a big chunk of the usage.
Regarding the chart above, the slope of the "line" for the fare is 5.4 cents/year and the slope for the price index "line" is 3.6 cents/year.