So let's get to pricing a bit:
actual tMP are at 2,999 and 3,999, while the iMP is at 4,999.
Let's shape off the 900-1,000$ that takes for the screen. I don't think Apple would sell the mMP at less than 3,999 for a 32GB/1TB base model,
Several dubious presumptions here just to start things off. First, the MP 2013 (tMP) pricing is stuck in 2014 era quicksand It is what Apple was charging back then. The "$3999" 8 core is largely anchored off of what 8 cores went for in 2014-5 era ( in addition to standardizing on the D700s in that configuration also). The next Mac Pro is extremely unlikely to pick D700 pricing model for the entry level GPU.
Second, "minus" screen is somewhat dubious premise also. Apple doesn't have to start at 8 cores like the iMac. Pro. There is differentiation that Apple basically have to do with iMacs that is not as high with a Mac Pro ( if they add substantive over dimensions to differentiate on). The iMac should be 6 cores. Puzzler why it isn't already, but probably will in 2019. The point is don't start with the entry iMac Pro configuration and simply remove the screen. Apple can start with a sensible start entry point for the Mac Pro and work from there. In the Xeon W line up 6 ( or even 4 cores. Intel's pricing on the Xeon W probably won't move much. If not at all it wouldn't be surprising to see a 4 core at entry spot.) is better starting place than 8. The RAM probably will start at 4x8GB DIMMs ( 4GB DIMMS don't match up to Intel W very well ). Apple could pull another cheesy move and just put in 3 DIMMs like the original "limbo under $3,000" configuration, but they probably don't have do if drop the 2nd GPU that that was saddled with. Finally on GPU a Polaris option that Apple used in an iMac probably would be the entry candidate ( avoiding the HBM mark up). There are only two Vegas to choose from and they are pretty close. If Apple wanted a 3 GPU line up again they can easily pick a Polaris ( or if wait long enough Navi mid range option. ). In short, stepping down on CPU + GPU will be more than just a screen "cut" to the price.
Third, depending upon what Apple does for secondary storage internally they may forego the entry SSD level of the iMac Pro. At Apple's crazy mark up prices for high capacities starting at 256 or even 512GB would be a substantive price cut. If there is a "cheap" 2.5 SSD bay or second M.2 slot then folks who need bulk capacity at sane $/GB could just get a third party drive . Everything doesn't have to be stored on one , and only one, drive.