Really depends how expensive you are! A one liter bottled water cost about 25 Nepali rupees . A good lunch at local restaurant would cost about 500-600 rupees. But if you go to tourist places you will spend at least 800-1200 rupees.
About $100 USD is enough for a month to buy bottled water, sometimes go out to eat in a restaurant, use public transport and socialize with other volunteers. One US dollars is about 125 Nepali rupees.