I disagree. There are rather huge gaps between Berlin vs. cities like Frankfurt or Munich. Living costs are much lower in Berlin, and so naturally, wages are lower, too.
I've also spoken to quite a few recruiters in recent years, and many of them were quite blunt that jobs in cities like Frankfurt, Munich or Hamburg usually get you 10-20 % higher salaries compared to Berlin, same kind of tasks, same skills required.
If you make more than 60k the living costs aren't much of a concern for you anymore.
Why? It's a function of supply and demand - for a long time a lot of people wanted to move to Berlin because it was cool, hip and cheap to live. So there was a steady supply of people wanting to work and live in Berlin, driving prices down for employers and since living in Berlin was (and still is) cheaper than Munich it was economically viable for employees to take a lower salary in Berlin and still come out ahead financially. Things changed a little since costs are converging a little, but as prices went up, salaries went up as well.
Berlin has certain advantages for tech employers:
* It is an expat-friendly city
* Nightlife is great, if you like the makeshift style
* It is the epicenter of the german hacker community
* Costs of living are still lower
All these are soft factors are opportunities for employers to haggle prices down.