Paizo has been quite open talking about their sales AIR, you can probably find some comments from Stevens & Mona if you google. The Amazon rankings are only useful if you keep checking them over a long period of time, day to day they're too ephemeral.
Distributors' published numbers are useful, but don't cover every channel.
Going in your FLGS and seeing what stuff gets the prime real estate and what gets relegated to a back-shelf can also be useful to get a feel for the market. Eg most OSR stuff is a lot more obscure off-line than online, Lamentations of the Flame Princess an exception. I remember I was pretty shocked when I walked into Orcs' Nest in late 2010/early 2011 and saw that Pathfinder had taken over the premium shelves (eye level, facing the door) from 4e D&D, with 4e in the shelves immediately below. It's stayed there ever since.
I don't think we need to know all of 4e D&D's exact income or RoI to know that WotC/Hasbro judged 4e D&D a commercial failure, anyway. You could just pick up the Menzoberranzan book and note that the lack of 4e D&D in it!