As predicted, an extra terabyte of storage for the Xbox Series X won't come cheap. You'll have to pay nearly half the price of the Series X just to double the amount of space you're working with.
Microsoft has been touting the Xbox Series X's 1TB expandable storage card for a while now. It's a proprietary PCIe 4.0 NVMe SSD which is top of the line right now in terms of speed. It's also very expensive, as Microsoft confirmed today that the official Seagate SSD will retail for $220. However, that's in line with what PCIe 4.0 tech costs for now.
For anyone who's looking at the Series X's base 1TB internal SSD (or the Series S' 512GB) and realizing it's not enough for next-gen's inevitably huge file sizes, biting the bullet on the external SSD isn't the only option. USB 3.1 HDDs are supported on Xbox Series S/X, just as they were on Xbox One.
Going that route will require some install location Tetris, though. Microsoft presented the capabilities of HDDs and SSDs with this table: