Yup stamp mills are one of the many things that have been both nerfed and WAAAAY over priced.
It should NOT cost steel for something that isn't all that useful... The only way you can really benefit from it is if you have 5+ of the running at the same time like someone else said, and that's like 4k worth of silver for such a small improvement.
I think JC needs to rebalanced a lot of the things. I appreciate the added complexity/difficulty so we dont reach developmental end game a week after we make a smelter, but this has gone too far.
At the very least make stamp mills either:
1: cheaper to make(wrought at the max)
2: better, as in faster/more inventory space/higher purity of items/lower hardness, perhaps have the stamp mill's purity increase over time like with turkey coops
3: both 1 & 2
4: have mine cart transport system like mentioned before me(and for the love of god dont make it cost 1 steel per tile

)
5: have the ability to put multiple boulders in at once