I think it's a bit misleading to go by nothing more than price per kg.
For instance, I can get a fantastic kava directly from a Tongan farmer. He normally sells his "standard mix", but he can also make a "waka"/special version of his kava that is 1.5 times stronger per gram than any decent Tongan kava. And it's obviously more expensive. So the price per kg might be higher but you end up using less powder + have a better experience.
Similarly, you can get pure lawena for much less than any kava you've ever bought. But you will need to use more powder.
So we are not comparing apples with apples.
Also, some of the more expensive kavas are really nicely peeled/cleaned and smooth. Cheaper kavas will likely be dirties/not super nicely peeled as peeling means extra work.