It’s notoriously difficult to get good numbers on publisher income, expense and profit — even nonprofits like PLoS only publish what they have to¹ — and so I’m always on the lookout for more data. If I had more spare time, I could dig out more information, but for now I rely on articles like this one (via OAN) from McGuigan and Russell at Penn State:

The Business of Academic Publishing: A Strategic Analysis of the Academic Journal Publishing Industry and its Impact on the Future of Scholarly Publishing

(Incidentally, in the unlikely circumstance that you’ve read this far and your eyes haven’t glazed over, you will probably like my oa.numbers and serialscrisis tags on Simpy, which is where I keep my collection of such references.)
Interested persons should, as the kids say, RTWT, expecially the nice readable introduction to scholarly publishing and the serials crisis; I just want to publicize this table of profit margins, comparing Elsevier S&M with the broader STM industry:

I am not going to pay over $100 for the Risk Management Assoc. data that McGuigan and Russell used, but I did download the UK Competition Commission report, wherein I found numbers supportive of the Elsevier figures in the table above.  The 2007 LJ Periodicals Price Survey says that commercial STM publishers’ profit margins were “around 25 percent on average” for that year, so the figures for “all periodical publishers” would seem to include a variety of non-STM publishers.  Even so, Elsevier’s science and medical division has a clear and commanding lead in the price-gouging stakes.
They also have a clear lead in market share. In one of McGuigan and Russell’s references (a 2002 Morgan Stanley report that you can get in pdf format if you have half a clue about search), I found a table showing the proportion of the STM market (measured in number of journals and number of articles) enjoyed by a range of publishers. With a little digging (in the filthy muck of commerce, at that; you owe me, loyal readers!) I discovered that Bertelsmann is part of Springer’s original name and they now own Kluwer Academic Publishing (as far as I can tell, most of Wolters Kluwer’s journals except for Lippincott Williams & Wilkins) under the rubric of Springer Science+Business Media, and that Wiley bought Blackwell a few years ago.
With that in mind, here’s an abbreviated version of the Morgan Stanley table of data:

Although those figures are from 2002, the 2008 Library Journal Periodicals Price Survey estimated that

the top ten STM publishers pulled in 53 percent of the revenue in the $16.1 billion periodicals market in 2006

so the bottom line doesn’t seem to have changed much.
Mind you, I don’t mean to imply that we should launch another boycott; reigning in Elsevier’s profit margins and/or market share would do little to offset the serials crisis. The only answer to that, in the long term, is Open Access, because it scales where Toll access doesn’t. No, this entry is not really about OA at all, it’s just a little kick in the shins for my favorite Greedy Bastard Publishers.

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¹ I’d link to the GuideStar reports but I can’t get them: I registered, but they haven’t bothered sending me the verification email, and until they do I can’t use their search.  What is this, amateur hour?