There was an interesting article including figures about publishing DRM-free pdf on loud thinking in 03/2006 :
http://www.loudthinking.com/arc/000580.html
"Then we spent a day building a dead-simple shop in Rails that would take $19 from your credit card and give you a PDF.
That scenario worked well for Getting Real, which was all short essays that you could print out if you liked or even read on the screen without too much distress. And in just three weeks since its release, we've sold over 5,000 copies."