My College Canteen — selling items on high rates

I will not mention the name of my college.
Please tell me what to do.
The person in my college canteen is selling items at a very high rates.
He sells 10 rupee maaza for 15 rupee.
12 rupee coldrink for 15 rupee.
And students are buying it.
I made a complaint to the principal of my college but he did not took any actions on it.
What should i do now?