I want to go to China with 2 KG of gold. I know I must declare. But how much the cost is?

If you carry more than 50g gold, you should declare to the customs and pay for the duty. It's ratio is 10 percent. However, you should know that the rule is based on "reasonable quantities for personal use". I am afraid that 2kg is beyond the reasonable quaitities for personal use. I guess you may have trouble when you bring 2kg gold into China.