[inglor_cz]

"Murdering or enslaving the population of the conquered country is a lot cheaper as a way to maintain control of it."

Depends on what you need the country for.

Nazis crushed the Czechoslovak state and executed any guerrillas caught fighting them, but they didn't mess with ordinary Czech workers, who were needed to keep the factories running. Mistreating qualified workers would decrease total industrial output and harm the German war effort.