honestly, i think if you upgrade an item, you should only have to upgrade it once to match your character's current level. so how about this, every time you level it up, you pay resources based on how many levels you've risen since you've last upgraded it. so, if you crafted the epic at level 10, and you've upgraded it at 15 it would have the same base resource requirements at level 20-25, or 45-50. your crafting skill and perks will reduce the costs too.