So I discovered by chance that Edward Young's three commentaries on Isaiah have an amazing discount right now: 65% off. For example, https://www.logos.com/product/8082/the-book-of-isaiah-vol-1-chapters-1-18
So, let's see 10.99 x 3 = 32.97.
But then I found that I can buy them as a collection of three. Unfortunately, the discount is only 16% off. Yes but, it sells for 29.99!!! 
https://www.logos.com/product/3637/the-book-of-isaiah
Now these figures include an academic discount. The prices above represent $6 off the three volume collection, and $18 off each volume when bought separately! So the individual volumes were even more expensive before the academic discount.
So all this has left me wondering: is the 62% sale actually a good sale or can I get a similar price normally on the three volume collection? 