I think one of the biggest problems with this is defining the product. Among different airlines that offer premium economy, the quality and pricing varies soooo much.

On a BD wide body longhaul, premium economy was an old style business class seat with less room and service than business class but VASTLY superior to economy class.

On a United wide body longhaul, economy plus is extra legroom and nothing else. Same seat, same food, same service, etc...just a few more inches of seat pitch.

Air France premium economy is somewhere in between the BD and UA products above.....

IMO, premium economy varies too much from one airline to another for a price comparison site to be very helpful. One would be comparing apples and oranges.