I have no idea what the prices will be in 2020 or 2025. There are so many things that can change. However, I am mildly optimistic and base this optimism on the fact that there's a lot of potential for increasing the supply of kava within the next 3-5 years. For a start, across the Pacific, despite the current boom, people still grow less kava than 10 years ago. At least that's what the Tongan ministry of agriculture and people in Samoa noted last year. Furthermore, over the last year or two lots of people started planting more kava that will hit the markets in 2018-2019. Additionally, lots of big producers that currently sell to large, pharma wholesalers might decide to start selling kava to recreational users. I will give you one example. 3 weeks ago I met up with a guy who grows kava (among many other things) in Samoa. His people currently sell kava for a low price to a German pharmaceutical company (they sell 1000-1500 kgs per month apparently). He's read about the high prices of kava and they are wondering if they should perhaps start selling kava to Western drinkers and whether they should actually plant more kava instead of planting more of their other crops. It's pretty good kava (no chloro, pretty nice effects) and apparently many other people in Samoa and Tongan are in a similar position (currently sell to the Germans, but might decide to sell it to the recreational market).
We also need to remember that one of the reasons why kava has become more expensive is that most big buyers have become more careful and demanding when it comes to quality. This is why the German companies buy their stuff in Tongan and Samoa (no risk of tudei). This is why the prices have not increased by as much in NZ as they have elsewhere. Yes, they have grown quite substantially, BUT in many cases vendors aiming at the diaspora markets have preferred to decrease the quality of their kava instead of increasing prices. As noted by Judd, it's almost impossible to find a Tongan exporter who wouldn't be mixing stems and peelings with their kava these days. Even some of the bigger, seemingly more serious exporters do this shit and are not even hiding this, e.g. see:
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