We usually plan on $75 a day just for "shopping money". This does not include food, snacks, activities, etc. - only stuff we want to buy. Some days we spend less, some days we spend more - but planning $75 for each day we are there works for us. So, on our 10 day trip last December, we had $750 of "shopping money."
However, the stuff we spend money on isn't really typical souvenir type stuff. After all our trips, we don't really need any more of that kind of stuff!
We like to shop and spend lots of money in the World Showcase shops - especially Japan! We also spend quite a bit in the African shops at the Animal Kingdom and at the home decor shops at Downtown Disney. Most of what we buy is stuff for our house. We like to get several nice things on each trip. That way when we look at them, they can remind us of our trip - even if they don't have Mickey on them or say "Disney World" in big letters. My favorite purchase so far has been a beautiful wooden bowl from the Animal Kingdom. It has incredible African animals carved around the outside edge - elephants, zebras, giraffes, lions, etc. We also spent quite a bit on Christmas decorations on our last December trip.
If you have kids, or if this is your first trip, you will probably not be able to resist all the typical Disney stuff - MouseEars, t-shirts, pins, posters, etc. I would still say about $75 a day is a good amount - unless you have several kids. In that case, you might want to set a certain amount per kid per day.
We always try to build plenty of "shopping money" into our budget. It's no fun to be on vacation and see something you really love and not be able to get it!