The only dual-core 28W Coffee Lake part is interesting, because at 3.0GHz, it has a higher base frequency than all the quads (2.3/2.6/2.7GHz) and a decent turbo at 3.6. Though priced at $304, Apple could get a custom underclocked SKU, for instance a 2.2/3.6GHz, which could bring that price down. That would also have the benefit of avoiding the bad optics of the cheaper dual core having a higher base than any of the quads.
The 15W chips aren’t any cheaper; the I5-7360U used in the base 2017 nTB is also $304, and the 2014 base mini uses the 1.4GHz i5-4260U which was $315. Note that the prices I quoted are all Intel’s 10,000 qty tray price, but I have no idea what discounts kick in with a qty of millions; guessing maybe 30-50% lower.
Apple wouldn't be able to get the volume required to get a good discount for a part that only goes in a Mac Mini though. It would have to be a part that was going into a MacBook Air or the non touch bar MacBook Pro - or - gasp - the Retina MacBook.
They'll have been warned of the private roadmap ahead for Intel so I dare say their CPU choices will be more informed than most analysts.
It probably makes sense that one of the unseen limits on the lifetime of the Mini could be the deal they struck with Intel to ship the Haswell CPU that goes into the 2014 model. Apple have to continue to produce spare parts for 5 years after the model goes off sale so you do wonder if Intel are discontinuing the Haswell mobile CPUs in 2023 and that has forced Apple's hand?
Whatever Apple decide to do it has to be with a view to using a common component to get the volume discount they need to make a price sensitive Mini work.
I believe it'll come down to whatever replaces the MBA in October with a side view at what becomes of the 21.5" iMac base model when it's time for a refresh.