Outside of Labor Day weekend, September is probably going to be a slightly less crowded time to visit (theoretically!). If that's the case, I'd expect G+ to be whatever the low-end price is at that time - perhaps still around $15. It's really based on crowd levels, or at least that's how it appears. Of course, there could always be an overall price increase - tough to predict. And I'm sure you know, but you won't have to buy it everyday of your trip - maybe one of the parks won't be as important to have it and you could save a few bucks that day. But we'd probably estimate $20 per person per day, and then hopefully be glad it's only $15.... Have a great trip!