I've not priced any potential trips that were a $400 difference or even close to that much. So far it seems to be roughly comparable to total cost of buying individual C+ upgrades for all segments under the old C+ deal.

FWIW, to me the extra inches of legroom are certainly worth the (pre-2016) cost. I don't eat the snacks and don't drink so the legroom is pretty much the entire deal for me. I would have to make other arrangements if eventually they price C+ at several hundred dollars more than Economy...the leg room is worth a lot to me but my budget is generally tight...

P.S. And as others have point out, Delta has not used the phrase "Premium Economy" or "PE" in any way as far as I've seen. They call it "Comfort+" or "C+" quite consistency as far as I can tell.