This is so individual it is almost impossible to answer. Things to think about.
Will your Longhorn have a car? If so, getting to HEB to buy snacks for the room will be easier than one who takes the bus (free to students) but is limited in how many bags they can carry back. If they have a car, they need some money for gas.
How many times will you visit? We typically took our Longhorn to Target and HEB when we were in town and stocked them up on our dime with things like snacks, drinks, toiletries and cleaning products.
How frugal is your Longhorn? Some will take the free bus and some take Uber. Some eat out at Whataburger and others at restaurants downtown. Some will go to cheaper theatres for a matinee while others will go to the more expensive places. Mine hate Starbucks but some go there everyday.
Are you including clothes in the spending money budget or are those separate? We cover all clothes separately because I have non-shoppers. If I had shopaholics, I’d handle differently.
Will your Longhorn be in the party scene? If so, they will be buying alcohol somehow- whether using their own money or part of the “allowance”
Are they picky eaters? If so, the on campus meals may not appeal to them and they will eat out or snack more.
Will their class schedule or EC schedule conflict with on campus meal times? More eating out or more HEB trips.
Will they be in a sorority or fraternity or Spirit Group? Some meals covered in dues but a lot of extras like OU weekend, formals, Round Up.
If they date, will your son pay for his dates? Or expect them to go Dutch?
Is Spring Break covered by you or them? ACL? SXSW? Texas Tribune Fest?
We initially came up with a budget based on how many times we thought it was acceptable to eat out, go out, etc. I thought the amount seemed high compared to my own college experience but found it was about right for my kids. They know people who get a lot more and people who get a lot less. If they overspend, it comes out of their money. We can still make exceptions when we feel like it is warranted.