Hi Robin!
Thank you for sending in your question to the Walt Disney World Moms Panel!
Since you will be staying offsite, you don't have to worry about lodging. That just leaves tickets, money for food and snacks, and souvenirs. I can't help you with how much to budget for souvenirs. I will simply say that Disney sells everything from cheap trinkets for a couple of dollars to very expensive jewelry.
For park tickets, you can get the latest prices and options for
Walt Disney World tickets at this website. To estimate how much money you need for food, I recommend visiting the independent website AllEars.net. At All Ears you can see the menu, including prices, for all of the Table Service and Quick Service restaurants at Walt Disney World. Needless to say, your food budget will be greatly impacted by the number of Table Service restaurants you dine in versus Quick Service versus eating outside of Walt Disney World.
By the way, since it your first visit, I recommend picking up the book "The Complete Walt Disney World" by Julie and Mike Neal. It is a great book for first time visitors as it is full of information without being overwhelming. It is also a beautiful book full of color photographs. It will give you a good feel for everything there is to do at Walt Disney World.
i hope this helps! Please write back with more questions!