For all (german-speaking) people, who still don't know it:
In the german magazine c't
you can find a construction manual for a low-cost roboter-arm (costs between less then 100 Euro and 150 Euro).
you can see the roboter arm in action (it is shown during a discussion).
you can find a preview of the article. And under the "Links" -tab you find a video, where is shown how to create and build it, the DXF-files for creating the parts of the arm, useful software, etc.