If you add-on from Disney, they are the same as whatever they are charging for new points. (Currently, they are offering a break between SSR and other-DVC options). On the open market -- best price.
I believe all points are the same price through Disney, but it is the dues that are different for each resort, I believe. At least that is what OUR guide told US.
Probably will be cheaper. When you buy resale, price per point is negotiable between you and the seller. After the two of you agree, Disney has a chance to exercise its ROFR. If Disney declines, the contract is yours at whatever price was agreed to.
Keep in mind that buyers usually pay closing costs when buying resale. That will add to your price per point. Disney pays closing costs when you buy from Disney.
Price per point is not negotiable when you buy directly from Disney. New members can only buy SSR from Disney at this time. Only existing members can buy add on contracts at the sold out resorts from Disney (if points are available for your use year).