I'm not sure that cost is the biggest problem. PoE splitters with USB ports can be had for ~$20. Though, relative to an RPi, I guess that is somewhat expensive.

Ex: https://www.amazon.com/gp/aw/d/B019BLMWY0 (note, I haven't actually used this product).

A quick ebay search shows some that are ~$5, so I'd imagine it wouldn't be all that hard for an experienced board developer to get the price lower at higher production rates.

Here's another one that's a brand you may have heard of and it also does multiple voltages too. Only $15. Can even get it as a kit if you don't have a Poe switch.

https://www.amazon.com/gp/aw/d/B003CFATQK/ref=mp_s_a_1_1?ie=...

Those cheap eBay items you reference are likely just sending power over an unused pair. Actual standardized PoE has all sorts of other cool perks. I haven't studied it heavily but I wouldn't say they're quite the same as the little dongles with a plug off the side.

You can now get a PoE adapter for the Raspberry Pi: https://www.pi-supply.com/product/pi-poe-switch-hat-power-ov...

I'm glad to hear someone else is interested in a Poe powered board. I thought about making a pihat that did Poe extraction but you'd still have to use a short cat5 jumper for data.