Protip: for guaranteed profit from friendship ( lol )
"Farm" gift rewards from npcs that like potions and colorless cubes:
You can get mild brew hp/mp pots from Seria for 50g per item, the cheapest possible option
Second best are clearless cubes, at 100g per item
Make the most of this by keeping the npcs within the lvl they accept these cheap gifts
If needed, use other chars that still have those npcs within the lucrative levels