Diesel engines cost twice as much to buy in the first place..hence the price difference. Per cc the honda engines turn out more "power" than the diesels .Hence you can have a small ,lighter ,quieter,cheaper (engine to buy) turning a bigger pump.
The equivalent 21lpm diesel model would need to be twin cylinder,heavier,noisier and cost Three times the price.
I am a big fan of the 250bar 15lpm Honda petrols these are cheap enough to run and more versatile.
I am now going to get slated to death from the big boys
Q washers... your second ebay selection and ask him to build it with a ws251 instead of the ws201..it will be the same price. You might struggle to find enough water for 21lpm without turning the machine down.