So the expats spend their home country resources for 20-25 years to get educated and be qualified enough to leave it and contribute to a different country's economy, taxes. Now the host country didn't spend any of it's resources in it's education, nor will it contribute to their healthcare - which typically becomes a burden only once they are older, by when they'd have left the country. Still seems like a net positive to me. Just can't put a number on it though. (wrt the 30% tax benefit).