MERs and New Horizons
I skimmed the New Horizons DEIS, which said the most credible plutonium release from an accident would be one gram (incidentally, a 1-in-300 chance of this happening, which is small but not trivial). What was interesting to me is I figure the threat from New Horizons and MERs is about equivalent, since I doubt the RHUs in the MERs had anything like the crash safety protections given to RTGs.
The one gram release was given a 20% chance of causing a single cancer death.
Of course, this all depends on whether one trusts the Nasa estimates of an accident likelihood.