How much money do I need for this trip?

It depends on how long you stay. Souvenirs and other items are relatively cheap so I would say $25-$50 a week max. If you want to take trips outside of the city that will obviously be more. You can go on a weekend trip for about $50--$75. You can do anything you want to if you are a little more frugal.

I say 25 to 50 week also. Although hardly spent 50 during week. Weekends Changes depending where you want to go touring.

For the basic living, i'd like to say 10 dollars a week in India is enough. ( include laundry, food, water...) But if you'd like to take a trip at weekends, and join in a party, at least 50 dollars a week.

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 simple and save you could last on 10 to 20 euro a week, 50 a week would be a comfortable amount. But it really all depends on the individual, how long they stay and what they want to do. Hope this helps