Hmm. I can't help directly, as I've been self-employed all my working life and don't know Jack about salaries (I'm
very interested in the delights of a regular working life, though

).
A friend of mine works as a Junior Java Developer in northern Germany. He makes €2,200/month before taxes, health insurance & other duties, which results in a net salary of about €1,500. He works a fourty to fifty hour week. This, however, is surely on the lower end of the scale.
There is a number of commercial salary checking services available, however all of them I know about are in German. Examples:
http://gehalt.monster.de/content/ser...html?NOFRAME=1
http://www.personalmarkt.de/spiegel/
If somebody at monster speaks english, it might well be worth the €20 investment and take the test.
But, do tell more about your prospective Job. Like, what will your Job title exactly be? Will you be in a supervisory position of some sort? How many hours will you work there? What other technologies / programming languages do you speak? Do you have academic credentials? Germans are big on academic credentials. And, location location location. Wages in east germany tend to be generally lower than in the west, as the overall cost of living is lower. (As far as I know, at least). Some major cities like Munich, Hamburg or Cologne have very high rents, especially Munich; others, like Berlin or Dresden, are more affordable.
Another idea, you know
Xing? Why not sign up there, look for a group of company execs or web developers, and ask around? My experience is that people tend to be more talkative there.