Great information, thanks! Though for my 2ct you are putting too much weight into production costs: how much metal there is in this price class is mainly marketing because it's the perceived "sturdiness" of the camera, even if this probably isn't backed by reality.
Fair point. There's certainly a prestige factor.
That said, I wouldn't underestimate the additional manufacturing costs. I haven't actually seen any of the metal cameras personally, but I'm assuming that a single piece of metal makes up the entire shell or very nearly so. If so, then in addition to the manufacturing cost, you also have the cost of assembling the guts inside a one-piece case instead of being able to assemble the case in pieces around the guts. Depending on what their manufacturing processes look like, the additional costs could be considerable (or not), particularly if the milled design requires a lot of hand assembly where the plastic design does not. Obviously I don't know whether that's an issue or not.
That said, there's a pretty easy way to know with some degree of certainty. Compare price points and sales volume. If price differences map neatly onto materials differences, it is probably almost pure marketing. If volume differences map neatly onto materials differences, it is probably a manufacturing constraint. If price and volume map neatly onto one another, guess, and you'll be right about half the time.
And the primary reason for Canon leaving out the flash out of their ff cameras will be that's it's their tradition and thus they can get away with it: it's generating even more profit to make users buy a €300-€500 unit just to serve as a flash master than to leave out some metal. For this reason, I'm not so sure they won't add a pop-up flash in the 70d/7d2 if it's possible to include it alongside gps/wifi with no (big?) loss of signal strength.
Quite true, hence my comment to the effect of "They didn't think the target market cared". :-)
Edit: Hmm. It looks like I actually deleted that comment in the midst of rewriting that previous post. Oh well. But yes, that's almost certainly exactly the reason they didn't include a flash.