I don't find much difference in CoL between the Bay Area and Copenhagen. If anything it's slightly cheaper in Denmark (more expensive restaurants, but cheaper rent). If you're comparing median U.S. CoL, sure, but even in high-CoL areas like San Francisco and New York City, $100 bills seem rare, despite the fact that >$100 tabs at NYC restaurants are not exactly rare. The difference seems to be that Americans are wary of larger amounts of cash, so use a card for anything nontrivial (or among older people, a personal check), while Danes can go either way, depending on personal preference. I'd blame it on the difference in mugging rates, probably.