I don't think it is common. I just know it happens.
The process looks like this..
Sort of a good example because it happens to be a traveler going into the United States for vacation, 5 weeks, 1.5k cash, but still denied entry. They always ask for purpose of entry, what itinery looks like, funds, etc.
There's tons of backpackers, as a one bagger myself, I've never had trouble. I think its when you may have limited/no accommodations plus other factors: stories not lining up, limited funds, no/limited knowledge of itinerary, etc. That gets people in trouble. I think most people know where they are going, what for, and how they are going to do it.
Even as a US citizen I'm happy I'm not traveling internationally for a while though. Seems most people are free game regardless to CBP, citizen or not 🤮
But overall CBP looks at funds. If someone can't afford their stay, they become suspicious.
EDIT: Adding, they ask how much people intend to spend or call banks to verify