FaceTime, iCloud and iMessage in China
FaceTime, iCloud and iMessage are location-independent. The location of the sender or recipient does not affect the price you pay to use the service: free.

However, how you connect to the internet may cost. Using free Wi-Fi will be free, or you can pay roaming rates to your SIM card provider for cellular usage, or you can purchase a Chinese SIM card and pay a provider in China. This is payment for connecting to the internet, to use any service, not specifically for any Apple services.

You should not change your iPhone region.