Im not sure if Marcel and Rudiger will answer that kind of question
I would probably estimate around 10,000-20,000 copies overtime. But the more advertising, the more sales!
I would recommend Marcel and Rudiger try and get a column in a local newspaper around Spandau - I can see the heading "Re-live Spandau in the 1980s" or something like that (in German ofcourse) - I dont mean pay for an advertisement but see if the local newspaper is intrested in writing a column about OMSI. This will probably boost copies being sold around Spandau in general
But hey, these are just my own thoughts