We need good, smart social workers - there's a shortage going into the profession.

That may be, but what opportunity cost is it worth?

I can tell you that 1- having good people makes a huge difference in what they accomplish

How huge? In order to make comparisons one needs quantitative data.

while they might advertise that a $100 donation will pay for so many vaccines- it doesn't really mean that if you give $100 that many lives will be saved. It's not that simple. There is overhead, program planning.

The $1000/life GiveWell cost-effectiveness calculations take this into account.

Getting a little more money isn't going to allow them to expand into another city to start giving vaccines to a new population and they aren't spending every dime they take in right away, they need to keep reserves on hand. In other words, you can't really know that you are saving all those lives by giving that much money. You will know what your impact is as a social worker, though.

Donating saves an expected life. Yes, one's donation probably won't result in an extra life saved but it has a small probability of tipping the balance toward allowing them to expand into another city which would result in lots of lives saved. See circular altruism.