Adding Shipping Costs to Germany bumps the price to about 22 USD (currently, 21EUR is what the receipt says). Considering the number of pages in this book in relation to its price, it is slightly expensive compared to your average "normal" book.

When comparing to other education text books though I'd not be able to complain. Neither am I, as I still think it was worth it, if just to support the author and to have an addition to the bookshelf.