$4-5k per wafer (sounds very low to me) doesn't say anything about yields due to manufacturing error. The smaller a CPU is, the higher your yields on a wafer because each error will destroy fewer and fewer chips.
When you have only a few giant chips per wafer, each error becomes devastating, taking out a large fraction of the wafer. Errors are so common I wouldnt be surprised if it took them 5-10 wafers per working chip, especially if they dont do any binning or they didnt design the chip to fuse off bad sections like Intel CPUs do (a single i3/i5/i7/i9 line is usually the same exact chip, with cores that have too many manufacturing errors fused off so perfect parts become i9s, less perfect parts become i7s, etc).
No that's not how you do giant chips. Even at the scale of CPUs, there are enough bad parts that it's uneconomical to throw away chips. What Intel/AMD do, for example, they make 4-core chips. If one core doesn't work, they sell it as 3-core or 2-core system. If some of the cache doesn't work, they sell it as a lower cache version.
In the case of Cerebras, they have disable the bad blocks to get chips that actually work.
Sure, but you still end up with area that is critical and shared and doesn't yield to this strategy, and you also end up with defects that are bad enough that you still can't employ a wafer with portions turned off.
So, someone like Cerebras has to both make as little critical as possible and buy much more expensive wafers with lower defect rates and still get only moderate yields.
When you buy wafers, particle counts at sizes are specified, along with all kinds of other properties-- some determined through non-destructive testing (and can be used for binning) and some through destructive testing (and can be used for characterizing lots).
But remember the cpu market is extremely price sensitive and Intel/AMD have such huge volumes. Manufacturers of large chips don't necessarily need to worry about the extra complication of binning in order to yield.
Also, It does not mean 16nm is cheaper than 20nm... the smaller the feature size, the more the wafer costs. Also, this is aggregate revenue at the foundry, and is ignoring things like tapeout costs.
Not trying to crap on the tech, but it's not so simple as we can merely go to the fab and get small quantities of full custom, 100% yielding chips, for 6k ea.
That doesn't make any sense because errors are an inherent part of the manufacturing process. If errors in the manufacturing process make 1% of the area unavailable that means perfect yields could at most be 99%.
I would be VERY shocked to see 100% yield... theres only so much you can do on chip though this strongly depends on what your definition of a working chip is :-P
That means we'll need 200 such chips.
Why is the wafer so expensive ? Buying a manufactured wafer from TSMC ,16nm is I think is $4k-5k
And there are still a few nodes left. While cost per gate don't go down much, it could solve some of the external bandwidth issues.
But all those things are really expensive and very risky. Who knows...