is it legal for a surgeon to charge whatever he thinks is fair for a surgery, if not then what is the amount till which he can charge
The charge or the fees for a designated procedure is usually decided by the Hospital if the surgeon is attached to it. Now if he/she visits as a Guest Surgeon as per the request of the patients family, then the personal charges can be decided by the Surgeon. Although the charge is to be decided on the basis of the professional ethics and the average procedural cost. So its not like that a surgeon can ask a very large amount for a small procedure but usually its in average a charge which is affordable by the family and sometimes the surgeons from the point of view of humanity charges very feeble from poor patients. So this are very relative terms and nothing standardized figures can't be produced.
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I hope this answer helps. All the very best for your future endeavors!