It's true zinc is kinda brittle, but so is aluminum, and zamak is not just zinc just like steel is not just iron.
Many a conversations was had about polymer guns namely the glock although not the first certainly the one to polarize the material.
That material comes in many forms and while often with flexibility there are some that are extremely hard as well so not all is equal.
As far as cost goes there is no doubt that the HP C9 is built as simply and cheaply as possible but that does not mean it fails in it's design goals.
The slide MIGHT be heavier then it needs to be, well then it's over built, but there is no doubt that at least some of it's bulk is due to it's blow back design, what % of that bulk I do not know.
High stress parts on a HP are steel, so right material at the right spots.
You can't really knock it being cheaply produced when that was a design parameter, almost every new gun design follows the same path to material savings these days, striker fired, polymer, and MIM
Course people distrusted polymer, now everyone uses it, everyone bashed mfgs like Taurus who used MIM, now everyone's fav mfg uses it at least in some fashion so that settled that.
How much does it cost to produce a "top tier" glock? dunno seen estimates but nothing is concrete although im sure it's a lot less than it's $500 price tag.
Do we knock glock despite this even though they sell for more then some guns made of more expensive (all metal design) materials, No.. We don't
Honestly I owned a HP before the torture videos. Even I was shocked how much abuse they took, But abuse them they surely did.
The HP designers must have been pretty smart to get such durability out of such "bottom of the barrel" guns.
HP is not my first choice but brother there is a whole other bottom under that HP.