On the Official MNSSHP thread in the trip planning forum there was info posted that tickets go on sale tomorrow as well as the tiered pricing for the various dates. Here's a link to that thread:
You've probably got til the day before to decide. Since this ticket is non-refundable, there is no advantage to you for booking early, especially for September which tends to be rampant with evening rain and severe weather. Better to check the weather report and then buy.
"Following the changes made to the single day park tickets earlier this year, both party nights use tiered pricing, with different prices depending on the date." This was copied from the wdwmagic boards post on this topic.