The best prices really depend on what you plan to do. You can frequently save major $ by booking your land and sea separately. However, if you do this, you'll need to arrange your
trip insurance privately, not thru
DCL (to get it all covered). September is low season (after labor day) so it is a good time to get discounted rates on both the cruise and the land portion. There's no harm to booking early--if DCL discounts your cruise further, they will give you the discounted rate (you have to ask for it!).
The key to a better deal at WDW is what you want to do and what specials may be running in Sept. If you book the land/sea cruise package, you get unlimited park hopper tickets (and you pay for them). The issue becomes that you usually don't use them on the first or last day....you fly in and finally arrive at your resort in mid afternoon. Will you go to the parks that day, or will you do a character dinner or downtown Disney? On the day you're leaving, do you need park admission? DO you plan to do water parks? How about Pleasure Island? If the answer to even one of these is "NO", you can probably save $ with parkhopper plus passes rather than unlimited park hopper.
The last time we were there, the cost of a week's pass to Pleasure Island was $5 more than one night....
So...in my experience, booking the land and sea portions separately has usually been a better deal. Hope this helps.