The Holt McDougal Algebra 1 answer key is available for purchase from the publisher, Houghton Mifflin Harcourt, at HMHCO.com, for approximately $67, as of 2015. The teacher's edition also contains the answers to all of the questions in the Algebra 1 textbook.
HMHCO.com also offers an answer key for the Steps to Success workbook for approximately $53.
Amazon offers many teacher's editions for sale. Teacher editions of the Hold McDougal Algebra 1 teacher's edition cost between approximately $5 and $160. The price fluctuates depending on how new the edition is, if it is Common Core compatible, what region it is from and who is selling the book.