Nowadays as they become more and more expensive, less people are buying them since there are many other cheaper alternatives, and people who still buys them are real collectors so unlikely they will sell them. Thus less people buying means less people selling used. This would also mean that in order to make sufficient profit, Bandai either have to continue increasing the price while reducing the production numbers, until the point where nobody will buy them and thus ends the GX series.

Do notice nowadays you can't really find them anywhere cheaper than retail, even used. So best bet is to purchase from online shops before they sold out, which then you'd have to resort to ebay or forum which will be much more expensive.