Anything cheaper might be unusable , too heavy, not at all weather-resistant, waste of time.
Paint your outdoor plywood very well, all six (countem!) six sides, then make a drip cap for your top edge out of pvc pipe or something to shed the rain, keep the wall up off the ground with a cement brick or something and your outdoor walls should last a very long time.
Sometimes people tare down a house and don't want the old doors, do you have a truck? Put the two together and walls become cheaper.