Aussi doesn't just mean "also." It also means "as/as...as that" (Perhaps you recall "aussi...que..." meaning "as...as...", e.g. as big as me. So in this case, it literally means "as expensive as that."
I cannot buy so expensive a book was. So I'm not sure if your problem was with 'not able to' or the order of your 'so expensive' and 'a book'. I'd even be tempted in English to translate it as 'I cannot buy such an expensive book'