Is there a limit for expensive item I might bring with me in the carry-on luggage?

You can take expensive items of self-use amount into CN and there is no need to declare them. If they are gift or for sale, you should make a declaration if their total value exceed CNY2,000.
As for your second question, do you mean taking cash? If so, you'd better take less than CNY20,000 or USD5,000, for you don't need to declare this amount of cash at the customs.