Unmembership
At Chabad, we don't charge membership, but we still have bills to pay. The Chai Club enables our supporters to spread out their support throughout the year, keep us going through the dry months, and have more mitzvahs!
Habit of Kindness
Maimonides teaches that it's better to give numerous, smaller donations, than one lump sum, even though they add up to the same amount. The logic is twofold: (1) Each individual mitzvah has unique value irregardless of the size. (2) The more acts of giving we do, we nurture a habit of kindness within ourselves which pays dividends in every area of our lives.
How much?
A little more than you were thinking. The Rebbe taught us to always push ourselves, a little bit at a time. At the same time, try to pick an amount that you'll be comfortable keeping steady.