I'm not sure it's easy for many startups in any space to become profitable.
However, it is possible.
One open access startup that has met this challenge and come to profitability is the Public Library of Science: http://blogs.plos.org/plos/2011/07/2010-plos-progress-update... They are now making a great deal of money, which is presumably why organizations such as Nature Publishing Group are looking at replicating PLoS One.
My understanding is that they are making a great deal of money, which they reinvest in the organization. The linked post has a link to a document which includes their balance sheet. You are, of course, correct that they're not-for-profit.
However, it is possible.
One open access startup that has met this challenge and come to profitability is the Public Library of Science: http://blogs.plos.org/plos/2011/07/2010-plos-progress-update... They are now making a great deal of money, which is presumably why organizations such as Nature Publishing Group are looking at replicating PLoS One.
There is a lot of online discussion of business models for open access startups. Here's one useful guide: http://www.arl.org/sparc/publisher/incomemodels/