I assembled a complete (or nearly so) set of Warren Wiersbe's "Be" series by buying individual volumes when they were heavily marked down. Given the hugh markdowns on volumes of the new _Expositor's Bible Commentary_ now, I'm wondering if that might be the way to start accumulating a set of them.
This new _Expositor's_ is considered a revision of the first one, which was under the general editorship of Frank E. Gaebelein. The first volume of that set was published in 1979. This newest _Expositor's_ is under the general editorship of the well known and highly revered Biblical scholars Tremper Longman III and David E. Garland.
Most, if not all, of the volumes of the new set are marked down, at the present time, at Amazon. Most of the volumes are $5.99, a handful are $6.99, but two are actually only $2.99 each. The two $2.99 volumes cover some of the Old Testament's "Minor Prophets." The entire Old Testament set is currently $150.47 at Amazon, making each volume an average of just under $19. You don't have to be a math wiz to see how much that you could save under the current sale.
The sale is scheduled to end the 21st day of this month.
Here are links to the two $2.99 volumes:
https://www.amazon.com/gp/aw/d/B01N5Y1RDX/.
https://www.amazon.com/gp/aw/d/B01MYE76PD.