The charge for extra baggage is INR 250/-kg. I know this because I had to pay it while coming to Doha. You will get your boarding pass only after paying the extra baggage charges. You have to pay it at Mumbai airport only (or any other airport from where you are boarding from India)

Cargo allowed is 20 kg + 7 kg cabin baggage. And any bag in the cargo should not weigh more than 32 kg.