Ask ten injectors how they reconstitute a 100-unit vial of onabotulinumtoxinA and you'll get a surprising spread of answers — 1 mL, 2 mL, 2.5 mL, 4 mL — each delivered with the confidence that theirs is the "correct" one. They're usually arguing about onset and diffusion. What almost none of them are doing is connecting that saline volume to the only number that pays the lease: realized cost per unit.
Dilution does not change how many units are in the vial. A 100-unit vial holds 100 units whether you add 1 mL or 4 mL of bacteriostatic saline. What it changes is handling — how precisely you can draw, how much you lose to dead space, how many patients you can split a vial across before it expires, and how consistent your dosing is from injector to injector. Those handling realities are where cost-per-unit is actually won or lost.