Hm, appropriate value. Lets see....It's essentially fully armoured steel peasant who meanders and has slashing weapons and breathes fire. Maybe more blunt damage resistance compared to actual peasant? Also dangerous as a child, not just adult.
Then there's how it can't be stunned or knocked out or go on break or drink or sleep or eat (kinda like vampire) and have it's helmet wrestled away, so a baby one might be able to kill squads of goblins (though probs not the entire siege due whips) even without the firejet.
Oh yeah, and unlike normal armor which has to be replaced, this one's heals.
Given all that, I'd peg the base price as full exceptional steel uniform * however useful not passing out through exhaustion or pain or just not being there due break is * (1+ reasonable expected lifespan/reasonable steel armor replacement rate).
Now, I haven't run the numbers, exactly, but I'm sure just the first part would be worth more than half a prepared meal.
As for the second part, I'd suggest arena testing.
XD
I'll be perfectly honest: when I was imagining these guys, the idea was that each one would be basically a typical forumite, with specialized equipment that could not be swapped out for other equipment without a long process that would have the potential for damaging the mech.
The current design would make sense for a (rare/high-quality) war forumech, but what about the blacksmith forumechs and the farmer forumechs and the miner forumechs and the cheese-maker forumechs? They would likely be made of cheaper materials, and have implements on them more suitable to those jobs.
I understand that having multiple types of forumechs, all of them as different from each other as I am suggesting, would be exceedingly difficult and complicated.
That said, it would therefore be exceedingly dwarvenly.
Cheers, mates, and let Necrothreat choose its path as it will. No need to get derailed by automatons while there are more dangerous things to deal with. Maybe it is too early.
EDIT: Of course, I did mainly come up with the idea because it spites both death and blood in its very conception.