What are the costs of not allowing immigration?

Sir you are absolutely correct when in your editorial “Migration debate” November 5 you call out for more “fact-based analysis, not rumour-based angst”. You also say that “Any debate about immigration must begin with asserting the truth that immigration makes most existing residents better off” and though I gladly sign up on that principle, I must remind you that the most missing piece of evidence in the whole debate, is the analysis of the costs of not allowing immigration.