If we are sitting in economy and want to bring another checked bag, can we and how much money?

If you take your extra luggage to take the Chinese domestic plane, generally speaking, the fee is 1.5% of the full price ticket per kilogram. It means, if your ticket price is 1000 RMB for the full price, you need to pay 15 RMB for per kilogram for your extra luggage.
As for the International flights, different airplane companies have different regulations, you need to ask your airplane company to comfrim.