To be honest, this is the second time I make this suggestion. The first time was during CBT1 when I was plying assassin. I talk about a real small fee here. Like character level * 2 rupees to enter the warehouse. So, if you want to open the warehouse at character level 10, you will have to pay 20 rupees. And potion craft should cost the same for the “10-potions-craft” and base level * 2/10 ( 2 rupees for lvl 10) for the small one.
Here is my old suggestion: