Well, you can spend a lot or A LOT. Assuming you are staying at a Disney Resort. I believe all rooms have a safe. Also, you can use your room key for all purchases in the parks and at your hotel. My family usually buys breakfast food and we eat in our room. Pop tarts, donuts, etc. Lunch, my family of four spends about $30 to $40. Hamburgers, chicken nuggets, fries, drinks, etc. Dinner is about the same.
You can try a character breakfast. ($70 for four) This usually fills use all morning and part way into the afternoon. Sometimes we have been able to get by on two meals and a few snacks in between. My family usually plans on spending about $150 - $200 a day on spending money. This includes popcorn, ice cream, breakfast, lunch, dinner, and any other snacks you want. This also includes some souviners (sp).
Hope this helps.
Have fun and good luck.