I doubt any of us are having one cup of kava per day.
I buy it as rarely as possible because shipping costs are enormous for me. My last big purchase was a few hundred dollars of medium grind from Nakamal@Home. Before that, it was a few hundred dollars of micronised from Kalm with Kava.
My general pattern is half a teaspoon or so of micronised Kalm with Kava Pouni Ono a couple of hours after I wake up. Later on, when the day's work and exercise is over, I'll have two heaping tablespoons of medium-grind, quite often extending to four tablespoons if I have time.
Justifying the cost is easy: I had anxiety-induced depression and now I don't directly and indirectly because of kava. The direct benefit is that kava reduces anxiousness, but the indirect benefit for me is that it got me off the sauce and got me to exercise more. I was drinking too much alcohol with my high-stress job (I'm starting a complicated business in a difficult part of the world). Because I was drinking at night, I didn't see the point in going to the gym as I knew I wouldn't be recovering properly. Now I have a regular gym schedule and really nice pecs.
I live within my means. It's not hard for me to find a few hundred dollars every few months, but I do notice when it has left my account.