It would indeed have to be a five shot. David Clements was doing these as well as .44Spl's, running $1200-$1400. Far as I know he suspended his five shot double action work. The only other two I know of making cylinders for double actions are Hamilton Bowen and Jack Huntington.
That said, nobody builds a custom gun with resale in mind but no, its value won't be less than the work will cost. We don't build `em to save money either. If what you want is a custom gun, nothing else will suffice. If you want a .41Mag GP I can hardly see how a Super Redhawk, only ever available in .44Mag, .454 and .480, would fill that need.