This is the URL of the web site Starshollow mentions, but the interactive form for putting in a street name doesn't seem to work for me:
http://www.hvbexpertise.de/en/index.htmlIt doesn't show a map and always gives the same values at the bottom of the page, the ones for a "Good" area. However, without the map for locating in which category (Very good, Good, Average, Simple) an area falls, it's not much use.
But there are flyers you can download (in German) with the missing map with these categories and indicative prices:
http://www.hvbexpertise.de/de/download/download/index.htmlJust go to heading "Marktübersichten Wohnimmobilien Deutschland", and choose your town.
These are only rough indicative prices, I think you would get a more precise idea of the prices in the street you are interested in by having a look at (only in German):
http://www.immobilienscout24.deHow to:
1. under the heading "Kaufen" click on "Wohnungen" (for flats) or "Häuser" (for houses)
2. in the fields at the right put in the street and town, and specify a radius around that address
3. you will get all the current offers within that area. Normally, you can negotiate the prices down by at least 5%.