Honestly, we budget (and spend) about $300 or less. For my entire family of four. For a week.
Putting aside the cost of the hotel, food (including gratuities) and airfare, we don't spend more than an extra $50 per day, and usually less. That covers a couple of alcoholic beverages for DH and I here and there, Mousekeeping and/or bellman tips, and a small (~$25) souvenir per person, for the week.
We have stores at home and can order Disney merchandise delivered to our doorstep from myriad websites, anytime we like, so for us, spending any appreciable time shopping in the parks is a waste. Along the same lines, spending money on "add-on" experiences not included in our ticket price (e.g., dessert parties, Bibbity Bobbity Boutique, jet ski rentals, etc.) seems ridiculous when we already can't squeeze in all of the "free" stuff in a week's time, and are paying a fortune to be there already. Ditto for spending additional money on a car rental or taxis, when we already get it for free. Don't get me wrong -- we have an absolute ball -- but we find we don't need much extra money to do it.
Whatever amount you decide to bring, do try to keep to a budget and track your expenditures, so the charges don't sneak up on you!