Overweight baggage fees are usually in the 60-80 INR per excess kg range. 70-75 rupees (i.e., less than $2 US) per excess KG seems to be about the norm for excess baggage (whether the excess baggage is another suitcase or an extraordinarily heavy bag or some combination of the two). Very cheap excess luggage fees by European and American/Canadian standards.

A number of carriers in India will waive the baggage fees up to the international allowance -- as a courtesy if not as part of policy -- if you mention that you came from an international flight the night before. (Having a foreign passport and/or your boarding pass from that international flight and/or a USD-quoted fare may help avoid the excess weight fee even as it may -- although less common than before -- trigger a "foreigner fare".)