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.
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.
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.