Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon's orbit and basic physics) the individual masses of Pluto and Charon are difficult to determine because that requires determining their mutual motions around the center of mass of the system which requires much finer measurements -- they're so small and far away that even HST has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157; more observations are underway but we won't get really accurate data until a spacecraft is sent.
Good point. Pluto is such an odd case that it may not even be a “planet” in any sense of the word.