Hi Angela,
You are planning a trip at one of my favourite times of year to visit Walt Disney World!
My family and I visited during the last week of September and first week of October in 2009 and thought that this was a great time of year to visit.
The temperatures are usually very pleasant in both September and October with an average high of 90 degrees fahrenheit and average low of 66 degrees fahrenheit. Pricing will obviously differ depending on exactly when you decide to visit and where you will be staying but generally speaking, most of this period of time is classed as Value Season 2 or Fall Season. The rates will therefore be towards their lowest and you will be happy to hear that crowds tend to be very manageable during this time of year.
All of the parks will be decorated for Halloween from the end of September onwards and I would highly recommend that you purchase an additional event admission ticket to
Mickey's Not-So-Scary Halloween Party if at all possible during your stay. Whilst attending the party, you will be able to collect candy at various trick or treat locations around the Magic Kingdom theme park, see lots of Disney characters and villains in special Halloween costumes as well as watching Mickey's Boo-to-You Halloween Parade and Happy HalloWishes fireworks. It it a lot of fun for all the family!
I hope that I have answered all of your questions and wish you a magical trip!