Like everything in China, it is "negotiable".   In fact, once arriving in Beijing from abroad, and switching terminals with two suitcases  at 50 lbs each, the girl at the check in counter started the letany, and I was not in a good mood after a 16 hours flight, so just to see what would happen, to every word she said I replied "mei you money"  After about 10 minutes, and many complaining by those behind me on the waiting lime, she passed my two suitcases, gave me the receipts aand waved me on. 
But I have always paid between 100 and 300 Rmb for whatever the exess was.
And be aware, if you travel domestic connecting from an international flight, and both flights are on the same PNR (airline ticket voucher), then international weight limits apply on domestic leg also.