I've made plenty of conventional cutterheads in 2, 3, & 4 wing, with about as many gib styles. (Wedge, pocket, visegrip).
I have also made quite a few insert body tools for wood and metal working inserts. It is not as difficult as some here describe, but maybe it is if you have never done it. Lots of gotchas on manual machines.
The bottom line as Onepass mentions, is 'til you buy the inserts and screws, material for the tool body, and maybe account for some consumable tooling drill, tap, small em's with tapers) and cutting fluids, there is not really much savings.
I do like the idea of HR steel, certainly do not use CR. My choice when I have to buy the material is 4140 (4242) prehard. But honestly I'm usually in too much of a hurry when the notion bites to make one, and I have used quite a bit of junkyard mystery metal as well. Usually some sort of heavy shafting.
We've had the argument on here before, but I do not like stress-proof for stuff with a lot of machining, especially assymetrical features. It moves, even if it is "not supposed to".
All the insert and brazed tooling in this stack was made here. There is a matching female set.
smt