$50/oz, plus whatever energy return pebax gives on top of that. Yup.
It's not so much the energy return as it is the extremely high compliance (compressability). Boosts also have super-high energy return, but they aren't as soft.
ZoomX likely allows you to transfer some of the cushioning that would otherwise be provided by the eccentric contraction of the quads (which requires energy) into the passive cushioning of the shoe. Basically it lets you run with a stiffer leg.