The longest parkhopper plus you can get is 7 days with 4 plus options and the cost of that ticket is about $46/day (I just divided it by 7 and figure that's for major parks and you may go to plus options on the same day, like going to PI at night or a water park in the afternoon or something).
At $46/day, you do indeed start to save money on your 11th day using a premium AP, but if you do more than 4 plus type things during that time period you start saving money even sooner.
Another thing to know is that Disney Quest is now included in a premium annual pass! It's not included in the parkhopper plus so you could start saving even faster.
Lisa
"You employ stone, wood, and concrete, and with these materials you build house and palaces. That is construction. Ingenuity is at work. But suddenly, you touch my heart, you do me good. I am happy and I say 'This is beautiful.' That is architecture. Art enters in."
-Le Corbusier