[valicord]

No, it's always "per unit that you use". Units that you buy and not use are wasted, so that's would be the wrong way to compare.

Dressing up an obvious idea with fancy-sounding math formulas and pages of text is exactly how you get trapped by someone good at selling stuff.

[jdpage]

You're right, but I think you're less likely to buy extra at per-unit pricing (since you can always buy just-in-time), so you're less likely to run into a situation where you overpurchased at per-unit pricing. (Edited to note that.)