To buy direct from DVD, the minimum purchase is 150 points. Purchasing smaller contracts would have to be from the resale market. When you purchase resale, you would typically have to pay the closing costs (when you buy direct from Disney, they cover the closing costs), which are 300-400 dollars. Adding $300+ in closing costs to each 25 or 50 point purchase will SIGNIFICANTLY increase the cost per share.
Purchasing from Disney really is very easy. They handle the financing, the closing, they include the 2005 points (kind of a windfall), and you can book your next vacation within hours of giving your downpayment.
The financing can be over 10 years with no pre-payment penalty, so you can keep the monthly payment down, and drop additional funds against the principal in good months.