I don't have numbers but MIRI seems to be addressing x-risk in a way that's more amenable to donation than other means. The most salient theoretically preventable x-risks in my mind are asteroid impact, engineered pandemic, nuclear war, and AI. Of these, I can donate to MIRI where marginal dollars help pay for marginal researchers, but I'm not sure what effect donating to physicians for the prevention of nuclear war would have, and there are already giant agencies searching for asteroids and dedicated to fighting pandemics.