even if you supposed that the lives saved all occurred 300 years in the future, a reasonable discount rate would still only give you a couple of orders of magnitude.

Sounds about right to me.

For example, 1.05^300 = 2.3 * 10^6

Huh? You think 6 is "a couple"? I wish I had your sex life!

But 5% per annum is far too high. It discounts the next generation to only a quarter of the current generation. Way too steep.

Which is nowhere near enough.

Double huh? And huh? some more. You wrote:

a yearly donation of $1 saves an expected 10^18 lives

You imagine (conservatively) that there are a potential 10^18 lives to save 300 years into the future? Boy, I really wish I had your sex life.