How would the combined masses of the Centaurs or the scattered disk compare to the combined mass of the asteroid belt? I'm interested in finding the combined mass of the Centaurs and the scattered disk. I looked around but I wasn't able to find any estimates. This seems to suggest that either the combined masses of the Centaurs or the scattered disk are negligible in comparison to that of the asteroid belt, or nobody ever thought of making an estimate. All I could find was that the asteroid belt is approximately 0.04 moon masses and that the Jupiter Trojans are approximately 1/5 of the mass of the asteroid belt. My feeling is that these combined masses should be way more than that of the Jupiter Trojans.  

Thank you in advance, and yes, I tried Wiki.
According to Sheppard et al. (2000) "A Wide-Field CCD Survey for Centaurs and Kuiper Belt Objects", the total mass of the centaurs is likely around 10-4 Earth masses.

Gomes et al. (2008) "The Scattered Disk: Origins, Dynamics, and End States" estimate the mass of the scattered disc as between 0.01 and 0.1 Earth masses. (Going by the abstract, I don't have a copy of the book to check the details.)

As you noted in your question, the asteroid belt is around 4% of the mass of the Moon, or about 0.0005 Earth masses. So the centaurs have a smaller or similar mass to the asteroid belt (bearing in mind the uncertainties on the values), while the scattered disc is considerably more massive.