Yeah, right. Altough I'd love to open-source it, it is our business not a hobby and it was a lot of work. And still is, as we often have to fix it when Apple releases a new version of iBooks and breaks simple webkit stuff. ;-)
Usually we get an audiobook WAV and a epub from our customers and generate the SMILs and package them with the JS.
Most expenses go to the SMIL part, as you might assume if you have already created some. The packaging and the JS software is a fixed royalty-free rate, its license is bound to the epub.