Enter the CiloGear NWD 75L. It's billed as a Seven Summits pack, and the cost is likely representative of the fabric, among other things. Dan McHale does packs in full Dyneema at an enormous cost, so I can't say that the price is out of line.
Cilo Gear is a small company and on my list of Cottage Gear Manufacturers. From everything I've heard, they build absolutely bombproof packs, too. That said, $1,250 is a bunch of change for a backpack. But if you need it, you need it.