The last iMac was a much better value proposition. I believe there's a huge price bump for the new retina 5k version.
Not really when you consider the system as a whole. The top of the line 27" 1440p iMac costs $1999 as is. But the new 5k iMac offers better processor and comes standard with a Fusion Drive. When you upgrade the 1440p iMac to match only those two specs, its price bumps to $2.399.
So, for only $100 more when compared to the closest equivalent "old" iMac, the 5k iMac offers not only a Retina Display with four times the resolution (pixels), improved contrast, viewing angles, and color accuracy but also a much better graphics processor and two Thunderbolt 2 ports. I'd say that is a pretty awesome deal. And don't forget the new iMac can be upgraded with even better graphics and processor.
Even the base iMac with a Fusion Drive is $1999, and it comes with an even worse video card. Apples to apples.