That is a bit high for textbooks, most textbooks are usually "only" in the realm of $80-160. That said, math textbooks are usually far more expensive than others.
The excuse given for high prices is that there is a strong used book market for college textbooks, as most students sell off their textbooks at the end of the semester, so from the point of view of the publisher, they see only one sale in 5-20. Of course, publishers discovered that the "cure" for this is to release new editions of textbooks with minor updates (most importantly, reorder the questions in the question banks!) every few years to kill off the used textbook market and haven't lowered their exorbitant prices to compensate.
As an undergrad, I did have one semester where my total book costs were about $1000. Most of the semesters, they were more like $500.
Oh, and the prices usually go to lining the publishers' pockets. Not the authors', not the schools'.