A: I sure wish I had been at that party so I could tell that guest how wrong he is. There's no such thing as an insignificant donation. As long as there are people and causes that can benefit from your altruism, there is no time like the present to give.
If you happen to run into that guy again, you might want to point out that small donations can make a big difference. A $10 donation to Habitat for Humanity buys a box of nails that will help to build a home for a needy family. According to UNICEF, $17 can immunize a child against the top six childhood diseases. The American Red Cross reports that $115 will buy a week's worth of groceries for a family of four.
Sure, the billions of dollars the Bill & Melinda Gates Foundation disburses have a global reach, but that doesn't diminish the fact that your charitable contributions, at any size, are helping someone somewhere to live a better life right now. No one can belittle the value of that.