$99 isn't the full price. It's $99 for every installation, in a commercial application. For an open source application it means tracking the license and making sure every user of the application pays the $99 if he hosts the application publicly.
Also you still have to support the code yourself, even if you buy it.
I'm not saying that this is completely unreasonable. What I am saying is that small javascript libraries are a very tough sell.
