Comichron has published some figures. Nothing for individual titles or anything, just total annual sales. And Comixology posts a weekly ranking, so we know what's selling (on Comixology, anyway).
As I recall, and my memory is foggy (and I could also just be wrong) digital sales jumped by massive margins from 2011 to 2014. Something like 24 million sold in 2011 to 76 million in '14. Since then they've leveled off somewhere around 80 million, and even saw a minor dip a year or so ago. They're not selling as much as floppies are, which are on an upswing if it can be believed.
But I have no idea how inclusive this information is. The Comixology numbers might factor in the DC and Marvel versions of the app, or it might not. It certainly wouldn't include stuff like kindle, and/or whatever other options are out there. Or distribution/storage expenses. And I don't recall how extensive the Comichron data is either, or what apps it tracks.
Its not a lot of information to work with in any case.