I believe 25 is the least amount of points you can purchase for an add-on; you don't really have to purchase in increments. You can get 25, 37, 48, etc., but you have to buy at least 50 to get financed through Disney and 100 to be able to take advantage of Magical Beginnings (although some members have previously posted that they were offered MB for an add-on of less than 100 points).