Seraphs (60 + .03 * 2390) = 131
Zhonya = 120
Void = 70
Large Rod = 80
Blasting Wand = 40
Items Subtotal: 441
Runes: ~26
Masteries: 22 (at lvl 18)
+Runes/Masteries Subtotal: 489
% Bonuses
Archmage Mastery (5%): 25
Dragon 1 (6%): 29.3
Cassiopeia 500 Stacks Passive (30%): 146.7
Grand Total: 690
Somehow I can't get to 730. Perhaps some of the % is cumulative, but in what order?
League Wiki says AP ratio is 50%. The tooltip calculates your AP at .50 ratio to be around 590. That's close, but not exactly, your AP not including your passive. I wonder if it's not applying your passive AP bonus.
http://leagueoflegends.wikia.com/wiki/Cassiopeia