I was going to say something similar to lkg4answers. The students will figure out a way to live within whatever means are allowed. The key is to set some clear expectations up front so you are on the same page before they go off to school. I have a freshmen. We agreed to pay for tuition + housing + meal plan + books and supplies ($800-ish for this year, as I recall - ordered on Amazon) + flights to/from home X 2 + Ubers to/from the airport when flying home X 2 + school-related activity fees or dues (intramural sports) + the occasional special event (like a concert)
My son pays for anything he wants to eat outside of the meal plan (including Starbucks) and other places he wants to go. He has to decide whether it’s worth an Uber ride or use public transport or chip in with a friend for gas and parking. There is PLENTY to do on campus so it’s not like they NEED to go anywhere for the essentials of daily life. He uses $ earned over the summer and over the holiday break. He seems to spend his $ on dinners, concert tix, and Target snacks (I do send him care packages. 
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Other costs to consider in terms of whether you wish to subsidize, have your kid pay, or cost-share based on their interests and what you as parents are comfortable with:
-Starbucks
-Ubers - can be $30 one way to Beverly Hills, for example
-Spring Break
-season football tickets [football games are not free to students; other athletics are]
-activity fees like ultimate frisbee, soccer [about $25-$30 per sport per season; ski club requires a ski pass]
-fraternity or sorority [could be $5k a year for dues] - NOT essential to be in a fraternity or sorority to love USC
So I think the point is, the budget for these extra things really depends on what your kid expects to do outside of studying and how much you want to subsidize it.