Living costs in most Indian cities and towns are normally low (especially in small cities). Although cost tend to be generally very higer in matropolitan cities like Delhi and Mumbai than mentioned below, it will give you a rough estimate about how much you should budget for the duration of your stay in most indian cities/towns:
Accomodation: Rupees 2,500 to 6,000 (i.e. USD 50 - 120) per month.
Meals: Rupees 20-100 or USD 0.5 - 2.5 (per meal in a reasonably good restaurant).
Transport: Rupees 50 - 100 (USD 1-2) for Distance upto 100 Kilometers (by Bus or Train).
Movie Tickets: Rupees 30-200 (USD 0.6 - 4)/ Ticket
Petrol: Rupees 48 (USD 1)/Liter
Please note that the above costs are approximate and may vary from place to place. Generally speaking a budget of Rupees 15,000 - 25,000 (USD 300 - 500) is quite good and will cover costs of occasional visits to nearby tourist places (including good budget hotels).
In addition to personal costs vounteer also need to pay a small administrative fee (of USD 50/month for International Volunteers and INR 500 for Indian Volunteers) after the placement has been finalised and once you give your consend in written for the vounteer program.