My pricing research is here: http://black.net.nz/old-k/old-k-prices.html#20Prices in Europe are likely to be a bit different from USA prices. I do not yet have enough data from K Zildjian Istanbul cymbals to calibrate the difference. I am generalizing from other price tracking research. And I haven't yet gone back and done a fresh analysis which includes weight for 20" cymbals in the predictive modelling. Again based on other diameters (18" where I have more data on K Zildjian Istanbul cymbals) and other sorts of cymbals (old As and 602s), there is a price advantage to lighter cymbals once you get to 18" and larger.
I'm surprised the new stamps have higher value than an intermediate. And it interesting how the prices have dropped since before the recession. It seems that prices 8 or 9 years ago for a 20" were much higher, if I recall correctly.