So $20 for 30mL... For retailers in any business, it is expected that they double their profit on any product..
I'm guessing they spent about $2 making that 30ml of juice. You spend $20 - about $8 of that $20 probably goes towards paying their overhead (rent on the building, paying employees, equipment they use to make the juice, etc...). The other $10 is their profit.
.... So yeah, $10 profit of a $20/30mL bottle of liquid sounds about right, as Zanaspus posted above