This ISBN number for locating the 10th edition of Penfield's Short Takes is 978-0205725472. That's the 13 digit ISBN number. When I last checked, the lowest price I had found was around $.99, shipping excluded. You should be able to obtain a copy for under ten dollars.

I will say that the sooner you purchase it, the better. Many of the textbook sites use an algorhythm that raises the price when more than one person begins searching for a particular book. Make sure your copy is the 10th edition.

This is the cover of the 10th edition. Note that many sites do NOT include accurate pictures of the used text being sold. So check twice on that.