The nature of the open source market has changed, and I've found that most people conflate free as in speech with free as in beer. As a result, people are picking up open source projects specifically because there's no cost involved, making support - or any kind of license or subscription - a very tough sell.
The only predictable way of selling services on top of an open project I've found has been to hold a piece back and make it commercial, which I think cuts against the principle of the thing. Nonetheless, eating and paying rent are important, so that's the way it goes.
One-off customizations are another matter, but the expectation is that because you gave away your code to begin with, you're going to work for a below-market hourly rate.
Disclosure: I've cofounded two end-user open source projects (elgg.org and withknown.com). At this point I could not be more jaded. It's possible the dynamics are different for infrastructure projects, but the state of OpenSSL et al suggest not.