For the most part, it depends upon how careful you are with your budget. Commonly, local trips, note print-outs, and books take up a quarter of the expenses. Generally speaking, the major portion of expenses technically results from the food facilities you avail of.
You can avail food from the hostel mess three times per day. Typically, the food quality can range anywhere from average to bad. However, the provision of food from the hostel mess is still quite acceptable, especially if you are a vegetarian.
Having said that, you can most likely find yourself making frequent trips to several food joints situated both in and out of the campus. Apart from that, you can pay a visit to the Civil Lines about once or twice a month at a minimum as well.
The food joints and restaurants situated in Allahabad are quite affordable when compared to other mega metropolitan or metropolitan cities. Which is to say, an average of about INR. 3000 to INR. 3500 can be quite enough to pass through a month if discreetly spent.
On the flip side, if you can manage well with the food facilitated by the mess, then you can conveniently take down your expenses to about INR. 2000 per month.