STRICTLY COSMETIC!!!
Actually, if you look at the angle of attack on that example, it will create LIFT.
Now assuming the spoiler uses a registered NACA airfoil, once that profile, chord and span are known, the amount of lift/downforce can be found fairly easily.
Two things to ponder with spoilers:
First, ANYTHING you place into the airstream will cause drag. Be it a spoiler, antenna, mirror, wider tire or whatever. This is called parasitic drag. Car designers spend great effort to reduce this.
Second, any lift, up or down, will also create drag. Newton's Law here. This is called induced drag. If you NEED downforce, you try to minimize this, but it is a necessary evil. No such thing as a free lunch.
Even by the time the Duster was designed car manufacturers were starting to pay some attention to drag, and lift. A Duster isn't too awful bad. Another thing to look at is what road racers were using back when these cars were being raced competitively. The T/A Challenger spoiler is a great example, just enough to disturb the laminar airflow. A small chin spoiler or air dam works well too, but their function is slightly different. The underside of the car is aerodynamically really dirty so less drag is created by blocking the airflow in the first place. More importantly, the dam forces all air going under the car to pass through the radiator first, and then evacuate down and to the rear. If a large amount of air travels under the engine there will be a positive pressure in the engine compartment, creating possible lift. Your firewall is also frontal area, it just has a big un-aerodynamic engine sitting in front of it. You want just enough airflow to create adequate cooling and thats it. Look at a new car with 250-300 HP like an RX8 or 350Z. Small light radiator with a tight duct leading in and in some cases out. Things they didn't pay much attention to in 1970.
Ever wonder how much slower an otherwise thoughtfully designed little car like a Civic goes with all the ground effect crap guys hang on them? Slower than cars raced in SCCA stock to be sure! And fuel economy must take an absolute dive.
