On my engine (IO-520BB) the difference in fuel flow between flying LOP and ROP will pay for the overhaul cost of the engine at TBO.
My current engine was overhauled by a company that gives one EXTRA year of warranty on the engine if it was flown LOP (very easy to see if they open it up).
So the price of proper engine monitoring is really secondary to the savings already made.
One major safety benefit: when LOP the engine produces no CO in the exhaust.
Carbureted engines can be made to run smooth LOP in some cases, use some carburetor heat to improve fuel volatilization.