For cheap walls, just rent a trencher that will cut an 8 inch trench. Trench to the depth you need. Add in a mat of rebar then fill with concrete. After you get both walls done, then dig out the dirt and pour your floor, Depending on your soil type you may need tie backs or temporary shoring between the walls. Place your pan deck across the top and cover with concrete. Dirt is the cheapest formwork available.