Walmart offers the Greenland Gardener 42" x 42" Raised Garden Bed for $21.97. Choose in-store pickup to dodge the $5.99 shipping fee. That's the lowest price we could find by $24, outside of the mention below, although we saw it for a buck less three weeks ago. Made from recycled wood and plastic, it's rot resistant and assembles without tools.

A close price: Home Depot has it for $22.66 with in-store pickup.