Not, how much do you think they will give.
Not, how much capacity they have to give.
Instead: if you are willing to be on the hook for the responsibility (if they say “yes”) and the disappointment (if they say, “no”), how much will you invite them to give?
And yes, I agree with Seth Godin (who inspired this post, here), it’s harder to answer than it sounds.
But, wow, is it important.