Dear Claudia:
Non-profit does not mean they do not make money. It is a tax code designation. PLoS has its financials on line as does most other publishers. Springer does not because it is a privately held company, but it is in all likelihood about the same as everyone else.
However, you have to see what the publisher owns and how it makes its money. Some publish scholarly journals, trade advertising based magazines too, and other properties so their profit picture would look different than just a STM publisher.
I would look at:
Wiley and Elsevier
ASM and Amer Soc of Micro as well as FASEB
PLoS
When looking at PLoS see if you can break out PLoS one from the start ups.
