Compensation between different cities

I would like to compare salaries between different cities in Europe. Is there a website that shows for example that typical salaries in Zurich are approximately 2x the salaries in London? How do recruiters compare candidates' offers from companies in other cities? E.g. if you were a recruiter at company X in London, how would you know that the TC you are offering is comparable to the TC that company Y in Zurich offered the candidate?