It's my understanding a geosynchronous satellite will put itself in a graveyard orbit just before it dies.
How many dead sats are in GEO or in neighboring graveyard orbits?
What is their total mass?
This is a difficult question to answer, so I'm going to take a stab based off of a few assumptions.
Okay, so given those, what are we looking at? Norad lists 804 objects in that orbital period range in orbit. Of these, here are some statistics:
I'm going to remove the large inclination satellites from consideration. That leaves 371 payload objects near GSO that are not being maintained in inclination, and thus are probably dead.
As for the mass of these, assuming an average of 1800 kg, which is the average dry mass of a satellite, that means there is about 668,000 kg of satellites in GEO that aren't being properly maintained.