You can assume that a masked material costs about the same as an opaque material that renders the same mesh with no masking, or more if early Z is important.
IF the mask operation happens very early in the shader, AND the graphics card knows how to optimize this, AND there are a lot of large, connected areas that will get masked (not drawn,) THEN a mask operation may sometimes be faster than it would be to paint all the pixels.
IF on the other hand early Z is important, AND the amount of masking is small, the masked drawing mode may be slower than opaque, and quite similar to the cost of transparency. (Masking should have the benefit of deferred shading, though, IIRC!)