Whenever I go looking, if an economy fare for the destination is X, then PE will be ~2X and J will be ~4x. I've found this to be pretty consistent for many years now.
I'm a pretty big boy (188cm/95-100kg), so the difference for me between Y and Y+ is not having to sit at an angle for the whole flight so my shoulders and arms don't intrude into the seat beside, not having my knees pressed up against the seat in front and being able to open up my laptop enough to watch a movie without having my chin on my chest. Oh, and in-seat power (less of an issue these days as laptop and tablet battery lives push 6-8 hours each even when watching video). So it's a significant improvement - the difference between a day of back and leg pain, or not, post-travel.
All J really offers over and above that as far as I'm concerned is a bit more shoulder room and the ability to lie flat (ish, depending on airline). For an additional cost usually measured in $thousands.
Obviously different people will measure the value differently.