Ideally, it would need to be self-configuring, though... I.e., something
like the IQRouter auto-measuring of the upstream bandwidth to tune the
shaper.
 
Sure, seems like this is easy to code because there are exactly two ports to measure, they can even be labeled physically "up" and "down" to indicate their function.

For reference, the GL.iNet routers are tiny and nicely packaged, and run
OpenWrt; they do have one with Gbit ports[0], priced around $70. I very
much doubt it can actually push a gigabit, though, but I haven't had a
chance to test it. However, losing the WiFi, and getting a slightly
beefier SoC in there will probably be doable without the price going
over $100, no?
 
I assume the WiFi silicon is probably the most costly piece of intellectual property in the system. So yeah. Maybe with the right parts being available, one could aim at $50 or less, without sales channel markup. (Raspberry Pi ARM64 boards don't have GigE, and I think that might be because the GigE interfaces are a bit pricey. However, the ARM64 SoC's available are typically Celeron-class multicore systems. I don't know why there aren't more ARM64 systems on a chip with dual GigE, but I suspect searching for them would turn up some).

-Toke

[0] https://www.gl-inet.com/products/gl-ar750s/