Orthonormal correctly understands where I'm coming from. I feel that I have very poor information on the matter at hand and want to collect a lot more information before evaluating the cost-effectiveness of donating to SIAI relative to other charities. I fully appreciate your point that in the end it's necessary to make quantitative comparisons and plan on doing so after learning more.

I'll also say that I agree with rwallace's comment that rather than giving an estimate of the probability at hand, it's both easier and sufficient to give an estimate of

The relative magnitudes of the marginal effects of spending a dollar on X vs Y.