It's actually easier than that. If you just want to upgrade an item (like a +1 sword into a +2 holy sword), it costs the total cost minus the original enchantment cost. Page 242, DMG.
However, if you want to add a second power to a slot-occupying item (as opposed to upgrading it to a higher form of the same power), you take the higher cost plus twice the lower cost [P. 243, DMG, plus errata]). However, even in this case, you take the total adjusted cost minus the original cost as what you'll have to pay.