Why 5D isn't "3D with price tags" — it's cost riding on top of a schedule that has to already exist.
"4D" and "5D" sound like they describe extra spatial dimensions, but nobody is modeling a fourth axis in space. Both terms describe additional data linked to the same 3D geometry: 4D links each model element to a task in the construction schedule, and 5D links cost and quantity data to that same schedule. The two are not parallel, independent add-ons you can pick either of — 5D is built directly on top of a working 4D link, because you cannot know when money is spent without first knowing when the work happens.