Those lips do help gas mileage. It's one of the easiest aerodynamic features that is also effective. Without the lip, the airflow just slams into the crossmember, then hits every rough object on the underside of the vehicle, making a turbulent mess. They can also help cooling by preventing high pressure from building up under the engine bay, which fights against the flow coming into the grill.
A brick can be aerodynamic, as long as the edges are radiused and the transitions are smooth, and you direct the air to places where it doesn't make too much turbulence.