The obvious cost is the need to render the scene geometry twice instead of once. An additional cost is that the deferred pass in deferred lighting must output diffuse and specular irradiance separately, whereas the deferred pass in deferred shading need only output a single combined radiance value.
Rendering the scene geometry requires doing it twice instead of just once, which is an obvious expense. Another cost involves the deferred pass in deferred lighting needing to separate outputs for diffuse and specular irradiance, unlike the deferred pass in deferred shading which can output a single combined radiance value.