Don't know, depends on the country.
I can give you an example from Europe. Switzerland is one of the highest income highest cost countries in Europe. If it weren't for different regulations mandating Swiss level salaries for foreign workers, import tariffs, taxes (and probably a bunch of other stuff I don't even know about), it would easily be possible to pay only 10-20% for a prefab compared to a non-prefab.
Basically the first sentence of the first section defines it:
"Das Entsendegesetz verpflichtet ausländische Arbeitgeber zur Einhaltung von minimalen Lohn- und Arbeitsbedingungen gemäss den anwendbaren schweizerischen Vorschriften."
Google translate gives this:
"The Posting Act requires foreign employers to comply with minimum wage and working conditions in accordance with the applicable Swiss regulations."