About 8 years ago I bought one of the Karcher 1750psi electric units for about $150. Works ok for what it is, a cheap, weak-ass pressure washer. It will clean anything given enough time.
I was in HD one day when they were selling their demo/floor models for a reasonable amount of money and got an 11HP Honda with AR pump. The hose and nozzles were missing, but I got it for $500 and went down the road 2 miles and got nozzles for $40 and 2 complete 50' hoses for $100. This is a nice size, probably more than most people need, but when called upon, you just throttle up, turn up the pressure regulator and hang on!!!!
I estimate that I can do 6-8 times the work as with the electric. So the 'cleaning units' or whatever that they refer to, GPM x PSI, is pretty accurate.
1750 x 1.5 = 2625
3200 x 4 = 12800 12800/2625 = 4.87 times the work
I think you get more than the because you have to pay attention with the bigger unit, have to keep moving, and because it is usually kicking back sand, rocks, and crap at you.