There is no opportunity cost deduction. Unless the doctor can show that he is losing money (amortized cost of office/staff overhead etc) on that $100 service, there is no deduction. They can't arbitrarily say they sell their services for a certain amount and deduct when they don't reach that amount.

If you have a widget that cost $50 to make/market/sell and you sell it for $100, then have a "sale" or "friends and family" discount to $60, you don't get to write $40 off your taxes. If you sold it for $40 you could write the $10 loss off of your taxes.