There are two things that I think we are talking about here. 1. OSS vs Closed Source Software and 2. Freeware vs Commercial software.
One can write OSS that is commercial or charges for related commercial services such as customisation or support or distribution etc. Redhat I think offers support services for it amongst others. Ubuntu is F/OSS while Canonical Ltd has vested commercial interest in it for it provides commercial support services for Ubuntu servers etc through support packages. So their developers who contribute to Ubuntu, would fall into this group.
One can also write OSS and not charge for it, eg GNU/Linux etc. Many others write software like GIMP, GNU XYZ ... etc
Closed Source Software usually refer to commercial software, and in most cases, commercial software are closed source, although the reverse is not always true. eg, practically all commercial software, including of course, the most beloved public-enemy M$ (Not mine anyway)... etc ... interestingly, it was said that IBM was one of the biggest software houses, that earned through expensive custom business apps that its (consulting) services arms delivered to their customers together with their "boxes".
The last group is slightly obscure, but at the same time common. This includes practically all the freeware that made DOS and Windows so popular amongst hobbists since the 80s. A good deal of these freeware are not open source, but are freely available for download as binaries only.
Strangely, one of the early concept of open source, I thought, was authenticity and security in that one could be 100% that an app was doing what it said it was doing, by having access to its source code for compilation and building. One could then scrutinize and weed out rogue code snippets that are stealing info. This point seem to have become overshadowed today by the high and noble "Knowledge should be open, so open source is the way" message. Color me old-school ... but sometimes I find it funny when I read OSS fanboys go religious or puritan with being open. It just makes me grin.
So if you are looking to be paid, you can do it either freelanced or under the clock, in a closed or open source manner. But doing OSS projects seem to be mostly freelance at the moment. Most commercial software house are still quite closed source.
Granted, if you have some good idea for a killer-app, you can also write Free OpenSourceSoftware and have a donate button on your site or something.
One thing though, if you look at the zaurus platform, on the reality side, there are approximately 10,000+ unique users on this site. And not all of them are active users. I won't go in the math here, but do some estimate so that you don't get demoralized with the wrong expectations ya?
Keep us posted if you cook up something ya?