It is per person, per segment. Our flight (DTW-MCO) was non-stop. To upgrade the three of us, it was $69 each...total of $207 (no additional taxes or fees). I do not know if you can only upgrade one segment if you have a connection. I have only upgraded a non-stop flight. I really liked it and I plan on trying to upgrade all of our outbound flights from now on.
Here's another question for all the AT flyers...
IF you do RAC at the resort, can you upgrade at that time? Also, what happens to the baggage fee in that case? Since you have to call BAGS in advance of check in to arrange payment, what happens if you get the upgrade, thus waiving your baggage fee?

(I'm just curious. I don't plan on upgrading on the way home...well, not this time.)