Diversity has proven benefits in many cases, I see it in my daily work. It is much more than just a number in a ranking.
> low-income regions (e.g. ex-Russian territory, Asia)
Asia? Low-income? These times are gone in many places. You need to update your knowledge. Diversity in your workplace could help :-)
In regards to the ranking - one of the most important rankings for international Universities is the Times Higher Education World University Rankings. Methodology section: http://www.timeshighereducation.co.uk/world-university-ranki... this factor is measured by the ratio of international to domestic students and is worth 2.5 per cent of the overall score.
German Universities have a big issue about ranking performances, case and point giving certain Universities a Government issued "elite" title.
At the remote university in Hagen I paid ~120€ a year for the student union. Every course is 60-100€.
At the university in Stuttgart I paid ~300€ a year and all courses were free.
There was a time I paid 500€ per semester extra, but they are long gone.