Overnight costs. Europe is 1/4 or 1/3 the cost of USA. The US is unbelievably expensive for housing. Campsites in the US are as expensive or more expensive as motels/pensiones/hostels in Europe. You get a private room with a bed and small breakfast and maybe shared bathroom in Europe for the same price you pay for a campsite at a park in the USA.
There's a lot of hiker/biker spots on the Pacific Coast, which cost usually $5 a night per person. And outside of that, if you stay in state parks, you shouldn't pay more than $20. Private campgrounds are of course another matter. The most I spent on my coast tour in 2006 was at a KOA that had "hiker/biker" accommodations for about $20.
Are motels/pensiones/hostels in Europe really that cheap?