Were I work, we work mostly with Titanium and Stainless Steel every day.
We also work with other Steels, and on occasion Aluminium.
I have to be honest with you all. When it comes down to raw material costs, for a piece of metal the size of an Apple watch is does not matter a stuff
Even if you were looking at say 1 dollar for Aluminium, vs 4 dollars for Titanium, so what, when the end product is $350+
Cost of raw material is a zero issue.
Machining time will be up. Depending on the grades. Stainless can be much worse than Titanium to machine, so we could class them on a par machining time and tool life.
Aluminium is as cheap as chips, and soft as putty in comparison.
Even weight wise. the amount of metal in the actual case is probably not much of an issue when compared to what's in the case and the strap.
Certainly not enough to notice when it's on your wrist.
A lot of the 'oooooh it's fancy metal' is just fashion marketing junk to justify a far higher price for something that costs only a few dollars more to make.
Unless you are into precious metals of course. But then we don't know, but we are assuming Apple mean solid sold.
But again, unless you have stripped an Apple watch apart and weighed the case we don't know. It's not going to be very thick metal