Almost every New American Commentary Kindle Book is on sale for $2.99: View them all here
Top rated ones include: Exodus, Numbers, Judges-Ruth, 1-2 Samuel, 1-2 Kings, Hosea, Nahum, Habakkuk, Zephaniah, Haggai, Matthew, and 2 Corinthians.
Many other books on sale:
The Betrayal: A Novel on John Calvin
Tributes to John Calvin: A Celebration of His Quincentenary (Calvin 500)
Recovering the Reformed Confession: Our Theology, Piety, and Practice
Martin Luther: A Guided Tour of His Life and Thought
Salvation by Grace: The Case for Effectual Calling and Regeneration
The Religions Next Door: What we need to know about Hudaism,Hinduism,Buddhism and Islam and what reporters are missing
Why Trust the Bible? (9Marks)
Who Is Jesus? (9Marks)
What Is the Gospel? (Foreword by D. A. Carson) (9Marks)
The Great Exchange (Foreword by Sinclair Ferguson): My Sin for His Righteousness
An Infinite Journey: Growing toward Christlikeness
The Ten Commandments: Ethics for the Twenty-First Century (New American Commentary Studies in Bible and Theology)
The Old Guys 
I miss the old guy – thanks for the info Bentley