The "per razor" price of brass seems affordable . . . but on a manufacturing scale of thousands of pieces, the cost savings to use Zamak is HUGE. Also to consider are the economies of scale . . . casting a few razor parts along with other items from the same pot of Zamak is less expensive as opposed to casting smaller runs of brass components . . . I'd be willing to bet the per-unit "process" costs for brass at this scale would be considerably higher than Zamak.
You could well be right! But still, the per unit cost for materials is really small (and probably comparable to plating if they do that right). If this were a highly competitive mass market for rather low-cost items, where 50 cents or a dollar was a real decision point for purchasers, I'd wholly agree. But that's not the situation with DE razors today.
Merkur could add on the extra cost and nobody would even notice. In fact, it could be advertised as a reason for choosing their product.
I've seen some painted parts on old items that have been exposed to weather, and the material looks a whole lot like Zamak. Not corroded after many years. I'm still suspecting quality control issues with the razors. Merkur have some documented QC problems of various sorts, and when I see that in one area I have heightened suspicion about other areas. If they farm out the casting process (I suspect they might), then it becomes a question of what they will tolerate from the supplier, who will cut corners where they can.
All that said, I'd still favor brass.
So, on a related tangent, why cast at all?
Why aren't there modern razors made like the Gem/Ever-Ready 1912 types, or the Gillette Tech, where the heads appear to be punched and stamped? Perhaps a head-mass issue, but part of that could be countered by handle weight, perhaps.