For most of my detailed cutting I've been using generic #11 craft knife blades and it seems the "wear curve" on them is rather aggressive to start with then kind of goes flat.
Like, a new blade goes through leather like butter but that lasts for maybe 4-5 decently sized cuts before it starts needing multiple swipes and then at some point it almost feels like the leather is bumpy or grainy where the blade isn't cutting through denser sections.
I don't want to be too wasteful nor do I want to be forever changing blades - how long do you tend to go between changes?
I used to wait waaay to long between changing my blades. That stopped when I ended up stabbing myself in the finger because I was pressing so hard because of the dull blade and the knife slipped.
Now I change my blade anytime I find myself pressing harder, or need to make more than two passes. Sometimes I just throw in a new one when I'm starting a project for someone else as stuff tends to come out nicer with a sharp blade.
+1 to the other user that mention stroping as that can help extend the usable lifetime (by a little).
But yeah, if you can get a big pack of them cheap online and just change them as you feel you need to.
I think that, having never really used craft knives much, I never thought about the costs or longevity. I've had a few of those little sets that come with one blade each of a load of shapes but not really any that came with a pack of replaceable ones. The knife I've got came with 5 blades, I never even realised how cheap 100 extra blades even was...