The current price cap has been in place since 2012, so prices will increase by 70% over the 19 year period from 2012 to 2031.
That works out to roughly 2.9% per year, which is a bit above the current US overall inflation rate of 2.3%, but less than the US housing inflation rate of 3.2% [1]. I don't have an opinion on whether domain prices should increase with inflation, but the increase does seem to be in line with it.