I just had a look at a 250g bag and asked myself if I would use 1/5 of that in a regular session. Maybe, maybe not, but I wouldn't be miles off. I would use about 1.5 litres of water I think. I'm no scientist when it comes to this.
Global Kava Exports might be a better bet, they are still expensive compared with what I pay in NZ but I can accept their prices, and the shipping is quick and free.