yup, one sugar cube of "neutronium" would weigh approximately 100 million tons.
one (perhaps dumb, slightly pedagogical, but bothersome) question: Since the only place you would find "neutronium" would be in the core of neutron stars, and since the material is kept super-packed by the conservation of angular momentum (L = L' from big star --> neutron star), wouldn't any amount of neutronium disintegrate if it were removed from the star?
i.e. a sugar cube of neutronium isn't spinning any longer, so angular momentum is no longer conserved, plus there's nothing to stop the neutrons from undergoing beta decay. So you would end up holding 100 million tons of "protium."
Or does the strong nuclear force counteract that possibility? blah either way, what i've described is hardly a practical scenario.