A top supercomputer can perform a GFLOP for around $3, in 2017.
The price of performance in top supercomputers continues to fall, as of 2016.
TOP500.org maintains a list of top supercomputers and their performance on the Linpack benchmark. The figure below is based on empirical performance figures (‘Rmax’) from Top500 and price figures collected from a variety of less credible sources, for nine of the ten highest performing supercomputers (we couldn’t find a price for the tenth). Our data and sources are here.
Sunway Teihu Light performs the cheapest GFLOPS, at $2.94/GFLOPS. This is around one hundred times more expensive than peak theoretical performance of certain GPUs, but we do not know why there is such a difference (peak performance is generally higher than actual performance, but by closer to a factor of two).
There appears to be a downward trend in price, but it is not consistent, and with so few data points its slope is ambiguous. The best price for performance roughly halved in the last 4-5 years, for a 10x drop in 13-17 years. The K computer in 2011 was much more expensive, but appears to have been substantially more expensive than earlier computers.