A high fixed cost for permanent stat increase might be difficult to balance. I have a better idea: not make the potion cost high, but as you drink more of the same potion, the effect of each additional potion will get lower.
I think a square root or cube root diminishing return is good enough. That means the total effect scales with the square (cube) root of the amount of potions drunk. For certain special stats that have a large influence, you can use logarithm.
For players that want to grind, they can get relatively high stats, but not too high that would break the game. (A mage with 100 melee attack is certainly broken.) And for players that don't want to grind, they can still benefit from the potion.