They will be $350. Do you want to know how I know? Lets see:
2600k -$317
3700k - $340
4700k - $350
4790k - $350
6700k - $350
7700k - $350
8700k - $350 (note that this will be the 6 core part)
It's not rocket science, intel are upgrading the top mainstream skew to 6 cores. So, imagine a 7700k with 6 cores, on an improved process, with a handy IGPU (awesome for troubleshooting). It will obliterate Ryzen and everything else.
I suspect clock speeds will be similar to the 7700k, possibly 100-200Mhz less if they want to keep it under 100W etc. All depends on how good the new 14nm++ process is.
Actually NOT the 8700K is the mobile 4core Kabylake part.