If it effectively slots into the lineup as a third stock tier 15" model, then you're probably talking:
- i9 processor
- Vega 16 or even 20
- 1TB SSD
The current model equipped with those specs is $3,449+ and I can't imagine it's going to be cheaper, so I am guessing it will start from $3,499. Of course at this point there are still a lot of unknowns, it could be a new design to debut the ARM transition, it could be an iMac Pro made mobile (mobile Xeon, ECC memory, enterprise grade storage, etc) - the latter would probably mean a starting price of $3,999 or even $4,999.