The cMP 5,1 can be cheaper. If, you want cheap now, I don't care about 2019 and future MacOS's and whether Apple will continue to support the cMP 5,1's... then, yeah, a cMP can be cheaper with "equivalent" power as current i7 iMacs. But, the current 27" iMacs are more power efficient, has a great screen already, comes with KB and mouse and has Thunderbolt 3, which is more flexible than PCIe, IMO. And, maybe TB3 is more expensive than just getting a GPU because one has to also get a TB3 eGPU enclosure. But, a TB3 eGPU enclosure in and of itself will give the user more flexibility down the road.... You will also get two GPU's to work with once you get a TB3 eGPU (the dGPU in the iMac and the eGPU).... which has been known to work in Davinci and perhaps, FCP X in future updates will be better at it...
I think eGPU's are more flexible because it's been known that one can connect two TB3 eGPU's in an iMac and basically have 3 GPU's to work with, where apps like Davinci can already leverage... And eGPU's are more flexible because you can use them with laptops, if, one would, say, get a TB3-equipped laptop down the road...
And, there is also something to say about how much easier a 27" iMac is to deal with, like, moving it around your room than the heavy cMP 5,1... even though it has handles, it needs deskspace or floorspace... with an iMac, since it's an AIO, it is easier in this way...
just my 2 cents... might have been 5....
