I'll bet its a 4-pack for the $145
edit:
yep,
This replacement battery is used for all AK-451 Models.
The battery pack consists of four D-size lithium batteries, and is field replaceable (90 hours lasting battery). Rated life is 5 years (50% of battery life) or one hour of use, whichever comes first, before battery needs to be replaced. The useful life time of batteries is ten (10) years.