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.
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.