On the other hand I would be quite hesitant to hire a developer to build a customized video player for tens of thousands of dollars if he was going to go on and sell that same code to others for $200 a pop.
Neither approach is right. There's nothing wrong with the write-once sell-many approach, that's basically all of non-custom software development. You shouldn't be charging for your time to learn, either. His 10 apps aren't worth $500,000 in any universe. Maybe you could argue that the entirety of the development was worth $50,000 and value each app at $5000. Maybe. I wouldn't argue that, but someone else could...