the payback will be very slow.
BUT doesn't mean don't buy a diesel
I like diesels mainly for the TORQUE! When the RPM's drop in thick grass or up hills, most diesels actually make more torque than they do at full speed.
Contrarily, a gasser will lose torque as the rpms fall, causing the RPMs to fall even more.
I have a JD1600 wide area mower. You can plow through 5 foot tall grass 11' wide!!! (Not that I recommend it...its abusive).
That machine has a Yanmar 4 Cyl diesel. It will smoke out belts before stalling the engine.
Plus, diesel is safer to keep on the truck, and the engines run cooler meaning less likelihood of a fire if dry grass builds up on the machine.
Lots of good things about diesels. Only major drawback is cost and additional weight.
Ken