A good commentary on James for under $5
Craig Blomberg and Mariam Kamell’s commentary on James down $4.27 on the US Kindle store.
"James (Zondervan Exegetical Commentary on the New Testament)" (Craig L. Blomberg, Mariam J. Kamell)
UPDATE: Holmes on 1&2 Thessalonians for the same price on US Kindle. Also recommended.
"1 and 2 Thessalonians (The NIV Application Commentary Book 13)" (Michael W. Holmes)