Have I been ripped off?

It is as good as a 30mp Bayer, but nobody said the images from the camera were as large as 30mp Bayer. Quality isn't the same as quantity (although in this case, the Foveon photos pack the same quantity of information in a fewer number of pixels).

Dpreview explains the background for the 30mp equivalence in this interview:

http://www.dpreview.com/news/2010/10/5/Sigmainterview

Now, even if I wouldn't trust the DPI value to mean anything at all these days, it does make sense that Sigma uses a lower value, simply because they need fewer pixels to render the same amount of information. A Sigma image printed at 180 PPI will look every bit as detailed as a Nikon image printed at 255 PPI (not 300, but not as far off as 180 may sound).