It all depends how long you are staying for, while volunteering you do get meals with the families you stay with but some may not like the food and want to eat out, generally India is one of the cheapest places you can visit, if you are purely just going to volunteer and don't intend doing any other activities which is doubtful in India then you won't need much at all, if you are the type to live...
Nope! When I was in India, our orientation group had volunteers of various ages!