u.t.:
There are four prime factors to consider when comparing engines.
1. Purchase price.
2. Fuel burn.
3. Weight: lower weight means lower Air Traffic Control (ATC) fees if maximum Take-Off Weight (MTOW) is reduced or more revenue from a higher payload.
4. Maintenance cost.
Item 3 is likely a no-brainer. If an aircraft receives original cert at xx MTOW with a heavy engine, and later a lighter engine is installed and cert, it will
probably be at the same MTOW - i.e. greater useful load.
Then, assuming it's not ZFW-limited, the later version carries more payload, more $$$ income, more than making up for higher fees.