The smallest allotment is 25 points. For 25-49 points you must pay in full. 50 points or more, you can finance with DVC. There's not set increment, you can buy 26 points if that's what you want. The points have to be in your same Use Year.
Assuming Disney has inventory that matches your use year, the price for the sold out resorts - OKW, BWV, VWL & BCV is $92 per point.
I don't know what it is for SSR if you only buy 25 - too many incentives and minimums for me to keep track of, LOL! (We have the right amount of point for us - maybe even too many now that 2006 dues need to be paid, LOL).