Great that you know what you wanted to do.

Choosing BSc hons. Mathematics is a very good decision.

Now ,fees for any government college and university depends from university to university and college to college.

Some universities take very less hostel fee like Banaras Hindu University(around 2500-3500 per month), Lucknow University .JNU has no Bsc course but its hostel charges are really less.

Some universities have hostel fee of around 1lakh so it is totally your decision where you want to go.

Feel free to ask if you have any questions.

All the best!