As MachTech68 correctly states, the value of this black Cube to a 'collector' would normally be diminished. On the other hand someone searching for an unusual modded G4 Cube may be prepared to pay a premium. Who knows?
There were many mods performed on G4 Cubes. One I recall dates from 2006 and installs a MacMini and transforms the casing black. The link which I bookmarked a few years ago is
HERE.
I have three G4 Cubes, but would never 'butcher' a complete one in order to mod it, but do appreciate the hard work and ingenuity that some are capable of.
I shoe-horned a 2009 2.26GHz Intel Mac-mini into the case of one of mine without destroying any of the Cubes original parts. Needless to say it looks original from the outside, and obviously the performance is transformed. PPC forum link
HERE.
If ever required I could easily remove the mod and resell as an original Cube to a collector.