All numbers here are via Brian Hibbs, via bookscan. He’s posted a top-750 sellers plus analysis summary of the US comics industry every year since 2003. Usually, manga makes up enough of that list to give a decent picture of the top of the market. OEL series are not included, not because of like/dislike issues but because my ultimate interest in analyzing this dataset is the potential interest of US figures on the Japanese market (with a sidebar interest in the effects of digital distribution). If you want to check how they or comics industry as a whole did, click on the link above.
A caveat: This data is for US stores only. It should be noted that this isn’t a comprehensive guide of Western taste, as it doesn’t address potential differences between the US and, say, France (where the comic culture is different and manga reportedly makes up a quarter of total comics sold). CNN reports that manga sales in Europe and the Middle East ($250 million) more than double manga sales in the US ($120 million).
Edit: Found the 2007 data here.
Edit 2: Added the 2013 data from here.
Edit 3: Added the 2003-2006 data from links found in the comments section. Note that, from 2003-2005, Tokyopop titles list “CLIENT” as the publisher.
Edit 4: Added the 2014 data from here.