You're right, it is volumetric. I have no idea how the dollars compare.
The figures are from Amazon (via the press), and they're a couple of years old, so I don't know where they stand now. 105 ebooks sold for every 100 pbooks. I've no idea whether they are counting those sold gratis.
Also, I don't know whether the expensive ebooks are selling well. I don't recall seeing those expensive ones back when I saw this article, and to be honest I haven't seen them since either because I haven't been looking. I ignore the kindle versions. I can tell you that Baen's ebook pricing has gone up but it's still less than paper.
One reason that I insist on epub (and without DRM) is that I can load an epub up into Sigil and fix many formatting issues. Bad OCR is another matter, but I haven't run inot a lot of that.