125 umbral shards to turn a named epic item into legendary sounds fair > You get an extra perk.
However, “normal” epic gear does not get anything. So logically, people will upgrade only named items OR legendary.
Or make epic items upgradable for 500 umbral shards where item turn into legendary and gets 1 random perk. That way, crafting would still make a sense, and you would have to grind a bit to get that extra roll?
Lower price of regular upgrades to 600 for epic item and add Upgrade to legendary option for more umbral shards.