I am going to guess $650 for the base model. Similar systems from small Chinese manufacturers cost about that and Valve can discount the price close to cost because they intend for this to be a complementary good to their software platform and OS rather than something to make a profit in itself.
Here is a Minisforum Elitemini HX77G for $659.
https://store.minisforum.com/products/elitemini-hx99g-hx100g
It has an 8 Core 16 Thread Zen 3+ processor, 32GB of Ram, a 1 TB SSD, and an RX 6600M.
The Steam Machine has a better and newer GPU, but it will save with the 6 Core 12 Thread CPU (even if Zen 4 rather than Zen 3+), 512GB SSD, and 16 GB of RAM to make up for it.







