Our Kindle Deals page is one of the best resources for deep discounts on Christian apologetics and other related books. Books on topics such as evangelism, discipleship, Christian worldview, theology, philosophy, church history, Bible study and more. The page currently lists over 200 titles and is updated daily. All titles are $5.99 or less although the majority of them are $2.99 or less.
If you like reading Kindle books, you should bookmark his deal page and check back from time to time, or follow his @Kindle4Christ Twitter account. (You don’t need a Kindle! Get a free reading app for your computer, tablet, or smart phone.)
Tim Challies has also collected 80 Kindle deals on a Word document. He has some great finds in there, as well (it isn’t focused on apologetics, although there are a few in there). I don’t know if he plans to keep adding to it, but for now they seem to be up to date.