Originally Posted by Sith_Happens
The "currency" could just be story progress. As in, once they've gotten to the level where they would have enough money to upgrade their weapon, they instead acquire some means of upgrading for free.
Yes, basically. However, giving them a budget gives them the freedom of investing it as they wish, something the OP asked for. Do you invest this 6000 destiny points (or whatever) in making your +1 scimitar a +1 keen scimitar, or into upgrading your ring of protection +1 to +2, or do you enchant your cloak to become a +2 cloak of Charisma, then improve 6 potions of cure light to cure moderate, and save the rest for when you'll be able to upgrade your full plate to +3? That's all in your own hands.