Premium economy / economy plus seem to scale pretty well. Most of the times I’ve booked it’s been about 10% extra in cost for about 10% space on the plane. E.g 34 inches instead of 31 inches of pitch.

To be clear, premium economy and economy+ are different things.

The former uses larger seats and is typically available on long-hauls. The seat is somewhat similar to old-style business class seats, or modern domestic first class seats. It is not priced at 10% extra, it is usually close to double.

The latter is priced at around 10% extra, but is the same old economy seat just with a couple of inches more legroom.

It would be nice to have to pay only 10% extra to get 10% more space, but the lack of scaling is by design on the airlines part, and textbook market segmentation.

Edit: this article has pictures showing the difference: https://www.cntraveler.com/story/whats-the-difference-betwee...

I once got an economy plus seat at a bulkhead and had to stow my laptop bag somewhere behind me. I'd rather squeeze into economy than pay extra for having no way to kill time but wonder whether it'll be stolen.