Do you have time to go with them anytime they need to see doctor in USA? You even need their help; can you take good care of them when they need it? Do you think about they need your help more and more as they get older an older? Money is only the small part, but the time is the hard part. Can you handle that? In China they can get help with little money, but not in USA. Make sure you can give them better life here before the immigrant application.
I am the only child too. I already got the green card for my parents. That give them and myself peace of mind. My parents have full medical coverage in China, they also have no financial problem, but they still want to stay with us. When they get older, they want to spend time with children, they are like children too. You can have time for just two of you, but everywhere we go is whole family (6 or 5 if my husband not going) including grocery shopping. I do not now if it is good or not.