I'm part of an open-source project commercially supporting two main libraries, a deep-learning framework, Deeplearning4j --
http://deeplearning4j.org -- and a scientific computing library, ND4J (Numpy for Java), to power it.
http://nd4j.org.
The amount of support companies are willing to pay for depends on how much they need the open-source library, and how complex it is.
There is a perverse incentive, of course, for some open-source projects to become overly complex so that users will end up having to pay them. A certain large open-source e-commerce platform comes to mind...
Companies can and do pay project maintainers for support and additional features all the time. We see this in the world of Java-based scientific computing, for example. I think you'll find that Java projects, because of their role in enterprise software, are more likely to become commercial than others.
Most open-source projects have enterprise distributions that vendors commercially support, like RHEL or CDH4... Those distros often include additional features. In Red Hat's case, they added a security layer that was a compelling value prop. In Cloudera's case, they often include supplementary packages like a management layer.
Support contracts usually indicate that there's a lot of demand and a company large enough to serve that demand. Hourly fees tend to correlate with a sole maintainer, and they can be very high.