I can't see how it would cost more than an A45/CLA45 or RS3. Right? Which are of course less than the M3/M4...
It will certainly cost more than the regular Golf R. Given the fact that this will probably be a limited production version, I'd say that VW would ask at least 10.000€ more, right? Ok, M3/M4 money is an exaggeration, but I doubt this will be a bargain!
I don't think the M3/M4 are all that special, being based on the F30 which is pretty common and that the Golf interior doesn't suffer in comparison with the F30 either. Put this drivetrain in the next Jetta and in the Golf wagon and I'd take either over an M3.
I'd say that to most of us, those who know the difference between WBA and WBS, an M3/M4 is much more special.