The Precast slab laying is quick, there is much time saving. In conventional RCC slab shuttering material is stuck for 28 days. This makes it cost efective. But care should be taken while filling the grooves of Precast slab. The Precast slab is laid in parts, so goove filling is important. Waterproofing material should be used for that. Now adays, screeding of 40mm is done above roof slab to avoid leakage.
In terms of material, actualy there is no saving, it's saving in terms of minimum wastage of steel.