A pot cost system that actually makes sense

With the current prices, magicians earn money by training with ultimates. Now, with double the price, they'll lose money. This means paying for leech will remove money, and finally, leeching would not be profitable. In the end, people won't pay for leech. Training the "normal" way would be more efficient money-wise for magicians. For other classes, they won't be hurt much.

Except magicians have no 4th job skills that do damage comparable to any other class over time.

We don't earn that much either, because every drop we get from leeching tends to go right back into the periods we're soloing. I have to leech 4 hours just to be able to afford 4 hours solo later. That's not a profit.

Potions aren't the problem there.
Changing potion prices won't change leeching either, the KMS experience change, that fixes leeching.