Wait... Was under the impression it also had an m.2 on the backside and one at the bottom next to the PCIe 16x slot?
Might have confused this board with another, I think Asus has other boards that use the same SO-DIMM m.2 board then.
Still, a bit surprised it takes an 80 mm board to fit a single 25mm wide m.2 slot, but oh well.
Indeed, at $400 it is pricey, especially as ITX competitors exist at the $250 range with the same chipset. Also, those ITX boards also have two m.2 slots, so why even bother with the C8I then?
The two m.2 slots on the riser aren't directly behind each other, but staggered. Likely has to do with PCIe signal integrity or something, but possibly something as simple as mounting screws/standoffs needing to clear the traces/slot on the opposite end. There are also fan headers and some other stuff on the riser card, but those aren't big enough to need much space. Also, remember that an m.2 2280 is noticeably longer than a SODIMM slot, so the card becomes taller simly due to m.2 slots needing to be above the SODIMM latches. So it's more like 1.5x 25mm wide m.2 slot + however tall SODIMM latches are + any clearance for the other ports on the card + height added by the heatsink.
I don't think it's known yet whether the audio board lives in a standard m.2 slot or if they're just using the physical interface for something entirely different.
I agree that $400 is pricy, but this is an ultra-high-end board, with VRM and chipset cooling way beyond the competition. Also, it has one of the best X570 VRMs, period. And a bunch of high-end features like thermal sensor inputs, LN2 mode, and so on. This is not a $250 board, and featuresets like this aren't seen on $250 boards pretty much ever and regardless of form factor. Given that the price floor for X570 is just barely below $200 for now, this is relatively "reasonable" in terms of how large a premium you're paying - normally ITX starts out at around $50 beyond the cheapest ATX boards, with ultra-high-end boards normally being $200+ beyond that again. This is just the confluence of an expensive platform (mainly due to the reportedly $40-ish chipset (compared to something like $5 for a traditional one) and this platform being attractive enough for motherboard makers to truly go bonkers on designing high-end boards. There's nothing comparable to the C8I on Z390, for example.