Zondervan’s NIV Application Commentary series is on sale, with each of the ebooks selling at $4.99–or less on Amazon.
I really liked Psalms vol. 1 in this series. There are a lot of really good volumes in NIVAC; I just picked up Genesis (now that I’m almost done preaching through it this fall-ha!), Peter Enns’s Exodus, and a few others.
All the Table of Contents now are hyperlinked, so navigating via Kindle or iBooks should be relatively manageable. You won’t get the same sort of search power you’d get in Accordance or Logos, but the price is tough to beat.