So, I know it's wise to purchase warm fuzzies and utilons separately, but it just so happens that I get a significant quantity of warm fuzzies from saving hundreds of lives. I'm weird like that.

Anyway, suppose (against all evidence) that utilities are ordinally intercomparable. Suppose further that the relevant chunk of my utility function is U(charity) = U(fuzzies) + U(altruism), where U(fuzzies) = ln(# of lives saved), and U(altruism) = (net utility of saved life to owner) * (my discount rate for the utility of strangers). Let's say the typical life saved by charities is worth 30,000 utilons to its owner, and that my discount rate for strangers' utility is 1/100,000.

So, if I save 200 lives, I get ln(200) + (30,000 * 200 / 100,000) = 65 utilons for me. If I save 2,000 lives, I get ln(2000) + (30,000 * 2,000 / 100,000) = 607 utilons for me. My original point was going to be that I do get diminishing marginal returns to charity, but apparently given my assumptions they diminish so slowly as to be practically constant, and so I will shut up and pick just one charity in so far as I can find the willpower to do so.

Hooray for accidentally proving yourself wrong with back of the envelope calculations.