medical exam費用大概是$200左右

It depends on where you live. Usually for exam only it costs about $200. If you want to reduce the expense, you can go to the public health clinic to get free shots. Many doctors accept the proof by the clinic. Ask your doctor before you do this. Since Med Exam result is only valid for one year, you may arrange the exam later in the process (say, when you get the fingerprint notice or interview notice). Explain in the cover letter for I-485, stating you will submit I-693 later. Otherwise you have to do it twice if the result expires before your interview. Good luck,