I think the key here is a huge userbase (in this case Joomla's). My experience is that people need help and want make changes in the extension so they fit their needs and they need help from a community i.e. the community build upon the extension even if the extension can be run out from the box. Not everyone can afford or can take the time to find 3rd party "cheap programming".
Of course, with GPL extension you'll always find people downloading them from other sources, but if you provide people exclusive service, like an extension community, updates, let people give ideas for improvements, service, documentation, then I think you get a good number of users to sign up. Personally I value my time as professional and can I see that I can save time to pay a subscription (or buy an extension) in front of to spend time to figure out how to solve things at my own, when the answer already are there or I can get help.
I think Rockettheme with it's community is a good example. Now their templates aren't free. But even if they were free .. I would sign up to their community because it's so much knowledge in their forums.
But I guess in the end, you have to try to find your own way to go and your business model depending how your extensions looks and how you think you can attract new customers. I think it's a lot of place for creativity here.
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