Cheapest way to build a garden bed?

untreated boards from a builder's merchant, two per side, screwed into a post at each corner. no base needed - cardboard on the grass underneath to kill it, then fill with topsoil and compost mixed. if it's eaten three afternoons it's usually because you're trying to level the ground perfectly. put the frame down, spirit level on it, dig out under the high corner only, and stop there.