I think it'll start at $6499, but it will be very generously equipped to start.
10 or 12 core starting processor, some version of Xeon SP (not Xeon W, because they want access to processors with more cores than iMac Pro and possibly to dual-processor options, which means the big socket). They may well get ahold of semi-custom Xeon SP SKUs that eliminate the 4 and 8 way multiprocessor possibilities, but are less expensive than fully-enabled Gold and Platinum chips. Intel is already doing this with the recently pre-announced 28-core hyper-enthusiast chip (that's not going to be an 8-way capable, $10,000 chip like the Xeon Platinum it's based on). It'll be interesting to see what Intel charges for that chip...
Navi based graphics significantly outperforming the Vega 64 in the top iMac Pro.
Probably 2 TB SSD standard, with 4 and 8 TB options. I'm hoping for M.2 PCIe SSD slots in addition to the standard T2-controlled SSD. They won't let us boot from anything but an Apple-supplied SSD, but they may very well allow user-added storage drives.
48 GB minimum RAM - Xeon SP is 6 channels, so 6x8 GB DIMMs is probably the minimum configuration.
How high will it go? I think it'll clear $20,000 easily in a maximum configuration, and how much higher it can go will depend on a few design decisions by Apple.
1.) Is it dual processor capable? If it is not, the maximum processor upgrade could be as little as a $5000 upcharge to get from 10-12 cores to 28. If it is, there will certainly be over $10,000 in potential processor upgrades, probably close to $15,000 (dual 28-cores).
2.) What will be the maximum graphics offering be, and does it support dual graphics? If all the graphics offerings are in entry-level to midrange workstation territory, we don't know what the Vega and Navi ranges will look like, but I could see the maximum (single) card being only a $1500-$2000 upgrade. If they offer either really high-end or dual graphics, it could be much more... HP offers up to ~$7000 graphics cards on the Z6, and it'll happily take two of them...
3.) What's the RAM capacity? It could have 6, 12 or potentially (in dual-processor models only) even 24 RAM slots. Apple could support only up to commonly available 32 GB ECC DIMMs, or they could also support much rarer 64 GB DIMMs. If it's a very conservative 6 slot design that accepts only 32 GB DIMMs, the maximum RAM upgrade could be 6x32 GB DIMMs - somewhere around $3600 for 192 GB (getting an iMac Pro to 128 GB is $2400 - this is simply 1.5x for 6 channels). On the other extreme, it could accept something like HP's $59,000 RAM upgrade to 1.5 TB
If all the upgrade possibilities are at their most conservative, it's still something like a $22,200 machine when maxed out. $6500+$5000 (processor)+$1500 (graphics)+ $3600 (192 GB RAM)+$5600 (8 TB SSD - I simply doubled the iMac Pro's upgrade cost to 4 TB).
If they support all of the craziest possible upgrades, there are theoretical configurations over $100,000!!! $6500+$15,000 (dual 28-core CPUs)+$15,000 (dual ultra high-end graphics) + $60,000 (1.5 TB RAM) +$5600 (8 TB SSD).
Will Apple actually sell any of those configurations? Maybe, maybe not - if they sell any of the real monsters over $30,000 at all, it'll be a very small number, almost entirely in Hollywood.
Another interesting question is what user expandability will be... What will Apple let us avoid the Apple Tax on? Here are some (hopefully educated) guesses... From most to least likely.
User-accessible RAM slots: 90% (most likely 12, reasonable chance of only 6, outside chance of 24 in dual-processor models). RAM will certainly not be soldered, but there is a small chance it's hiding behind "service tech only" screws or heat sinks.
Add your own internal storage: 75% (either 2 or 4 M.2 PCIe storage slots in addition to a required T2-connected SSD). The chances of being able to replace the primary SSD yourself are lower, and that will certainly require a part bought from Apple.
Replaceable AMD graphics (custom boards): 65% (either users or service personnel have a better than 50-50 chance of being able to switch graphics cards or perhaps add dual graphics - these will not be standard PC-type PCIe GPUs - they have to output over the built-in TB3 ports, rather than the card's backplane, and there will be NVidia-proofing).
PCIe x4 slot(s): 50% (won't be fast enough nor support enough power for graphics, but there's a decent chance of a slot or two for your audio interface without resorting to an external box).
Replace the processor or add a second: 30% (depends on cooling system - much higher chance OWC or some other upgrade shop unofficially figures out how to do it). If there's a dual-CPU option at all, it may well be factory-only, not least because it could be a modified motherboard, power supply or both.