If you need to make use of a BoB I wouldn't be worried about the cost of ammo!
Depends on what you mean by "make use of a BoB". You mean don't worry about the cost of *while* you're making use of it to "bug out" (i.e. during an emergency) or making use of it as in having one and preparing for an emergency?
I guess that during a Red Dawn With Zombies (RDWZ?
) situation, you need ammo whether you're having to trade your gold coins for it, pay through the nose for it, grab it off an abandoned shelf or whatever.
If you're going to have a firearm in your prep gear, thinking in advance of ammo price while gathering supplies ahead of time and availability before/during/after an emergency could certainly be a concern.