Overstay and want to extend my stay in Guangzhou airport

Hi, I have an 4 month multiple entry visa for china. a few day ago i when to Hk and i try to re-entry china they find me overstay for 17 days. I pay the fine which they want n issue me a 30 days visa. I planning to extend my visa for 6 more months is it possible? if yes, how much does it cost and what should i do?