So, I don't know if it was a slip of the finger or what, but UA has published some PE fares. I see one published on NYC-HOU (not EWR-HOU). They appear to be using a differential fare table similar to what's being used for F.

Looking at November flights, I see a fare family:

• SAA2AFBN is a BE fare (books into N) at $184 + taxes and fees ($198.20 all-in)
• SAA2AFDN is a regular economy fare (books into S) at $214 + taxes and fees ($228.20 all-in)
• SAA2AFPN is a PE fare (books into B) at $303 + taxes and fees ($317.20 all-in)
• SAA2AFFN is a F* fare (books into Z) at $406 + taxes and fees ($420.20 all-in)

So, compared to regular economy, BE is -$30, PE is +$89, and F is +$192,

Next is a W fare family at $234/$264/$353/$456 base fares, keeping those route differentials exactly consistent.

They're not published in many markets -- the only other one I've found so far in a spot check is NYC-CHI -- so I'm not sure how much to read into this. I do think it gives some insight into what will happen on domestic legs of connecting PE flights; they'll probably book into B for the cheapest PE fares and Y for the most expensive ones. Note that those fare classes are instant-upgrade eligible.

* or J, if you prefer, in the new era where F is a retired fare class. But they're still published as domestic first class; "JUA" is an F fare that books into the J RBD.