how much money to donate?

http://www.nytimes.com/2010/05/01/your-money/01money.html?emc=tnt&tntemail1=y

“He advised Hopkins that if he wanted to become familiar with the nature of Christian
belief, to begin by giving alms and giving to the point of personal inconvenience, so that
something that you wanted to do, you wouldn’t be able to do because you had given goods
to the poor,” Mr. Anderson said. “It’s not giving to the point of utter impoverishment, but
it’s not just giving pocket change either.”

i think this is a pretty fine answer: "to the point of personal inconvenience"