"Fair market value" payment is likely required by the hospital's financers, other creditors or possibly its articles of incorporation. Your hospital might do any of a number of things to determine it, ie) hire consultants, compare to survey data, etc. Obviously they can choose the consultants or interpret the data however they want to pay you less. I suspect most employers compare based off hours or units, as opposed to bottom-line reimbursement. So depending on your payer base it could be better or worse. Make sure you figure out how they get their numbers and what adjustments they make or should make to the inputs (ie, sick patient population, long turnover times, efficient resource utilization, QI measures etc) and hopefully you have anesthesiology representation at the reimbursement meetings.
As always, know that the administrators want to pay you or your replacement the minimum amount possible to get the cases staffed and billed.