Since I choose RMB ( Currency in China) to be my billing currency .I have to pay 5% more for my clicks .

How can I know that ?

That is easy the least cpc for content network is $0.01 USD .   according to the exchange rate today. It is ￥0.075 RMB.
But Google still charge me ￥0.08 RMB per click .So Google is using its own currency exchange rate .

So if your country’s currency is valuing against USD,you’d better use USD as your Google adswords account’s Billing currency