Interesting study by the European Commission:
Long-term Social, Economic and Fiscal Effects of Immigration into the EU: The Role of the Integration Policy
"simulation results suggest that, although the refugee integration (e.g. by the providing language and professional training) is costly for the public budget, in the medium- to long-run, the social, economic and fiscal benefits may significantly outweigh the short-run refugee integration costs. Depending on the integration policy scenario and policy financing method, the annual long-run GDP effect would be 0.2% to 1.4% above the baseline growth"