I have a problem with Aid potions. When crafting them, they charge me 120 coin. This is 40 more than spells like Bull's Strength, Endurance, etc., that are significantly more powerful.

I believe there is a reason for this. Aid Does stack HP wise

So 5 aid potions could potentially give you fourty more HP.

Cost of potions and wands aren't all the same per lvl of the spell . They seem ajusted to the efficiency of the item. Hence hold wands cost more than other lvl2 wands, etc...

Yes, that's exactly what LTS was saying. Aid potions aren't as powerful as other spells of the same lvl, and therefore shouldn't cost as much, and certainly not more.

Same principle as the hold wand being more expensive, only that it's not applied to the case.