It's less about the programming (it's been noted multiple times in the forums by the main guys involved that if you just want to program a classic architecture style, GBA is an excellent target with lots of tools and documentation) than about the principle of having a complete understanding of the platform. Basically it's about reviving the oldschool sense of "ugly" (by modern standards) tricks that make it possible to do amazing things in a limited architecture. The book takes you layer-by-layer through the architecture so that by the time you're done you hopefully have a level of understanding similar to the person who designed the system, and can go about making your own additions/hacks and swapping them among the XGS community. Basically Andre is drawing on his experience in facilitating self-learning to create a new generation of oldschool hardware hackers.
As for the price, nothing is set in stone but $100US is the target price.
edit: and when I say nothing is set in stone, that applies to a lot of the specs too... Andre still hasn't made a 100% decision on the CPU, or the feature set of the GPU, for example.