Can I pay for an extra bag on an economy flight?

I have just booked a flight from Chengdu to Guangzhou and I have two bags. One will be 20kg, one will be about 13kg, and the other is a piece of hand luggage. It says that you can only check in one bag. Can I pay for another one and are you able to give me a rough estimate of how much a 13kg backpack would cost? Thanks.