To be written
Enhancing is done to improve the stats on equipment. What's important to know is that it is the same across tiers (so an orient sword and a tidal sword of the same level costs the same to enhance), so there's no reason to wait for the next tier to start enhancing.
I'm still in the progress of creating a good formula, but I'm basing everything on the cost for Charms (which start at 90 silver). From there, everything follows the same progression; for instance, weapons cost 45 silver to start, and they're also half as expensive to upgrade in later levels.
Axe/Sword/Knife/Bow/Tome/Staff/Wand: 45 (50%)
Amulet: 36 (40%)
Plate/Robe: 22.5 (25%)
Ring: 18 (20%)
Chain: 13.5 (15%)