I decided to do a more thorough analysis.
First off, the inclusion of TITAN cards *really* throws a wrench in these figures, even when only including the GTX TITANs that were marketed as gaming cards - They're just way out of line with everything else in terms of both price and performance. That's why the first chart I'm going to post doesn't include them, so that we can draw some more realistic conclusions from the data, and not have it muddied up by Titans being so chaotic in terms of their cost and their performance increase over a previous card.
Secondly, the performance change figures I'm using, are all from TPU's "Performancy Summary" pages in the respective reviews of each card. I have deliberately not used the average summary however - I have only used TPU's highest resolution summaries, in order to eliminate as much variance due to CPU or game as possible.
Some caveats - Firstly, yes, there's no such card as an HD580. I meant the GTX580. Secondly, this chart is not adjusted for inflation. You'll need to do that for yourselves if you want more data. I've done it for a couple of examples below however.
You can, of course, draw your own conclusions, but what strikes me is that between September 2010 and June 2015, we saw a tripling in performance and an RRP increase of 71% - Accounting for inflation over the same period, actually only 55% over a little under 6 years.
Since then, we've seen performance jump to 210% of a 980Ti, but that's come with a price increase of 74%, and that's not over 6 years, it's over only 3.
That's at RRP for Reference Design cards and doesn't include TITANs. If I add in the Titan RTX, (Which TPU didn't test but Toms Hardware says is 6% faster than 2080Ti) then we've got 223% of the performance of a 980Ti, but with a 363% price increase after adjusting for inflation.
However you slice it, and even discounting the fact Pascal as an architecture really was a surprisingly huge leap forwards, the truth is, customers in 2019 are spending more money, for a smaller relative improvement than they used to see for their extra dollars, and the price increases are themselves getting steeper.