On the one side, if Paradox keeps producing unit pack DLC, it's because their data shows people buy them enough to justify having the researchers, modelers and texturers working on it.
Creating a unit pack takes some time. I'm not a good modeler, but I've done my share of basic Blender-ing, and it can be very time consuming. I have been texturing for quite a while and, while my ADHD helps me hyperfocus on that for hours on end, sometimes I wonder where did my day go, with only one decent quality job finished (not accounting for all the research).
So, yeah, it takes time, and time is money.
On the other hand, I think that, for what they are, they are overpriced. Sometimes the research is wonky, sometimes the result is underwhelming. Many can't decide what time period they're exactly portraying, and so you'll find Tier 1 sprites which look like 1400 foot soldiers, others which remind you of 1500 foot knights, others which look way too much like they're in the 1530's, and it's a bit of a mess.
The worst offenders, to me, are the Aragon and Naples early sprites. Side by side, they look like they inhabit totally different centuries, but on top of that, the Aragon sprite is, as far as I can tell, mostly made up, while the Naples one is a generic late Medieval spearman with a GIANT LILLY ON HIS CHEST. I'm quite certain that unit pack was rushed and very little research was made on the early models.
Quality (like in most Paradox DLC) is inconsistent across the board. For every Evangelical Units you have your Mediterranean Portraits.
So I try to only ever buy those unit DLC when they're on sale.