The ABV of a whiskey during its post-distillation maturation in its cask, or barrel. Part of the concept is that the consumer gets the unaltered whiskey at its fullest flavor, which can be diluted to individual taste. In most cases, the only difference is price and water between a regular strength and cask strength. Elijah Craig Barrel Proof is around 69 ABV depending on the amount of water evaporation (Angel's Share) from the cask. The ABV will vary from cask to cask so each bottling is unique. Regular Elijah Craig is bottled at a constant 47 ABV by adding water and reducing the barrel proof strength.
Whiskey Woman has an article further information on this subject and might explain why it cost a little more.
Our friends at Modern Thirst  took the time to rate some of the more common Barrel Proof Whiskeys.
And finally, the Men's Journal explains how to drink a cask strength whiskey. {Not that you need any help with that}