How much should a person give to help families celebrate Yom Tov properly, and is there a minimum amount that actually makes a difference?

The minimum is never the goal for holiday charity. The Rambam teaches that ensuring the poor properly celebrate Jewish holidays is the most critical mitzvah; neglecting it when capable is a great sin. Obligation means giving according to your full financial capacity, not just comfort. Ideal giving is 20% of resources, 10% is standard, less reflects stinginess. Small donations are insufficient for a proper festive meal. True giving demands effort and sacrifice, honestly reflecting what you can afford, as Hashem values sincerity above the amount.