Speaking of which, I am pretty sure there is a additional cost required to put an enchantment on a item that already has an enchantment. Yep, a 50% increase to put an enchantment on a enchanted item. Page 282 of the DMG goes into detail about this, in how a belt of +4 str and +4 dex is worth more then a belt of +4 str and gloves of +4 dex combined.
Since the belt takes up only one slot, compared to the belt+glove combo. It should cost more.