Just copied this to explain
While sodium methylate solutions appear to be more expensive than sodium hydroxide or potassium hydroxide, they can be less expensive in practice. First, since the solution contains methanol, it lessens the amount of methanol that must be added to the reaction. There are some savings from this. Second, it is usually possible to use less sodium methylate than the amount of sodium hydroxide needed to get the same extent of reaction. This is because of the loss of sodium hydroxide associated with soap formation. Finally, the increased soap formation from the use of a hydroxide catalyst causes a loss of yield that wastes a significant portion of the feedstock oil. Not only is the oil which goes to soap lost, but also some of the good biodiesel product is lost when the glycerin is removed at the end of the reaction. This is because soap increases the solubility of biodiesel in the glycerin phase.
Hope this helps
Paul