You counter that with:
Well, I have no information about that. What counts for me is the opportunity costs, i.e. what I could make on other engagements. I love my work and I love this company, but unfortunately I can't afford to lose money just to work here. So can we please concentrate about the value I have to offer?
By the way, German law supersedes any contract you might have so you can talk to your colleagues about salary. Reference in German: Was das Arbeitsrecht zur Schweigepflicht beim Gehalt sagt
Edit because of comments: Regardless of economics, which I'd be happy to discuss in another place, I found this approach useful to focus the negotiation on what matters: What you want and what you employer has to offer!
It's always possible that you can't come to a mutually beneficial agreement and that's ok - time to leave. But at least it counters the usual bs like but your colleagues ... or I wish to pay you X but we currently can't afford to ... etc.
Also, for people interested in a lot of theory behind salary negotiation, I found this very interesting paper.
