If your checked in baggage is more than 15kgs on Indigo Airlines flight, you can buy the excess baggage allowance up to 30kgs. However, the applicable charges are not based on per kg calculations. Rather, there are pre-defined slabs of 5kg, 10kg, 15kg and 30kg attributable to these charges. So, if your excess baggage is, say 7kgs, you will have to pay the charge for a 10kg slab.
Since Indigo also operates flights to international destinations, there are charges for Indigo baggage allowance, for more than the free allowance, in the currency of that country.
Earlier, Indigo had price slabs for only 5 and 10kg weight categories. Recently, it has also introduced 15 and 30kgs slabs for convenience of passengers who have to carry more luggage. You can check these charges here.Please note that these slab rates are prepaid in nature, meaning that you will have to pay for them in advance of your checking in time so that there are no last minute hassles.