I think it really all depends on your spending habits at home and what budget you decide on with your parents. I, personally, cannot afford to spend over $500 each semester -- I wish I could, but I just don't have that option. But there are a lot of ways to cut back if you don't/can't spend that much as well!
Remember that you're on a meal plan so, if money is running low, hit the cafeteria for meals! It's almost always open, and there's also the option of getting Meals in a Minute at other places on campus {which you don't buy with JCash or Dining Dollars, but which count as a meal block instead}. I also make a lot of my meals on my own {since I love cooking and lived in Wolman with a kitchenette -- dining dollars are great for that}, so I have leftovers and extra ingredients to last me for a while. I think eating out and ordering take out is where a lot of people spend a lot of money -- if you cut back on that, you'll have a lot of money left for other things.
It's also fun to go shopping in Towson or downtown, but it's easy to watch and make sure you don't buy something every time! Also, laundry is kind of unavoidable .. but Hopkins keeps it relatively cheap for us and you can use your JCash to pay for it, so it's convenient.
There are a lot of ways to have cheap fun too -- take the shuttle somewhere and explore {but be safe!}, see a movie that one of your friends owns instead of going to the theatre, hit the museum {which is free}, etc. You'll need money for a lot of things, but there are also a lot of ways to cut back.
Like Mandy said, I would take a look at what you spend money on now and what sorts of things {movies, eating at a restaurant, etc.} are a regular part of your routine. Consider those things and add a little room to move around, because living on your own you will obviously have more expenses to cover yourself than you do at home. That's the best way to gauge how much you'll need next year. :)