It took me a minute to wrap my head around the question at hand after the caps/yelling started emerging (I thought the question at hand was closer to what jens/keii were pointing out) but I see now-
like I recently got a support of 100 ink and the before-fees amount is $0.08 as opposed to the $0.12/13 that one would expect from the calculation in the OP. However looking at the purchase options above the $2 one, you start getting that "extra" ink, the 4,000+1,000 for $5 for example, and so on. Looking at some of those, the value of the ink gets closer to what we actually see.
$5/5,000 = .001
$10/10,500 = .0009
$20/22,500 = .00088
$50/60,000 = .00083
(and the one-time offer, $2/2,400 = .00083 as well)
So it seems the real value of ink is that .0008 value, as we can see in the several examples provided. But the purchaser eats some sort of penalty when buying smaller amounts. The way the shop presents it is like you're getting a % extra ink for your purchase, but I guess really you're actually just approaching the true value instead. Good to know prior to Inksgiving I guess- I think last time I just bought several of the mid-tier options lol