The Premier Pass is $722.75. I was told they couldn't do the upgrade at Guest Services outside the park, so I had to buy a ticket to get in the park first, which was $76. Once inside they subtracted the $76 and the $520 I had paid for the WDW AP, leaving a total of $126.75 that I owed.
So basically, yes, it's the total amount, less what you've already paid for your original pass. It was totally worth it for us, considering it was about the same cost as a 3 day pass to DL. I'm not sure it would be cost effective the other way around, unless you were planning either a very long trip, or multiple trips to WDW.