Ignoring my international flights ($1500) and other pre-trip costs (e.g. visa, vaccinations, travel gear/supplies),
my average daily expenses while in India were approx $42 per day (inc. trains, which admittedly I could have done in cheaper classes).
Indian Rupee (INR)
INR 1 = US$ 0.0256147
US$ 1 = INR 39.04
(update: as of mi-Dec 2011, the exchange rate is US$ 1 = INR 52.74, i.e. the rupee has gone down 25% in value vs the dollar in 3.5 years)