It might be (depending on many factors), but even if we assume this is the case, what makes you think that it will be comparable in terms of price/product positioning to a "mainstream" Skylake?
Knowing AMD (*cough* R9 Fury X pricing *cough*), if they do have something that handily bests a Skylake 4C/8T on their hands, they won't sell it for $350. They'll try to price it up there with an 8C/16T Intel CPU.
Remember, this is the company that is now telling investors that they're tired of being the cheaper solution (paraphrasing). This should be a very big clue as to what AMD would charge for a part like the one you have described.
Yes, that's a good question. Unfortunately we have no idea how AMD intends to position Zen, or at what price.
As for Fury, I think they had no option but to price it quite high, since it's such a huge die and thus costs a lot to produce. Also, AMD and nVidia have in general been seen as equals (even if nVidia has pulled away a bit lately). Therefore it's also easier to charge a similar price to their competitor's products at the same performance levels.
But in the x86 space, AMD is the underdog, so they have to price their products a bit lower. In addition, an 8 core Zen die should not be much larger than a 4 core Skylake die on 14 nm, since Zen will not have any iGPU. So the cost of producing an 8 core Zen does not have to be that high. In other words AMD at least have the option of selling Zen at a lower price than Intel's 8 core 5960X which costs $1000.
Now whether AMD actually will sell Zen at a lower price or not is another story. But if they don't and try to charge $1000 for it, then I don't think it's be a success sales-wise.
As for the technical performance claims vs Intel 4 core CPUs I made earlier, those are still valid though regardless (if the Zen guesstimates are true). But I agree the comparison to a mainstream 4 core Intel CPU will not be fair, if AMD decides to charge $1000 for an 8 core Zen.