The cheapest way to DIY an urban planter box is not to build a new wooden box at all. Start with a free or very cheap container you already have or can find locally, such as a 5-gallon bucket, storage tub, sturdy crate, old drawer, or food-grade barrel. These cost far less than new lumber and work well on balconies, patios, and small city spaces.
Drill several drainage holes in the bottom, because good drainage matters more than a fancy design. If the container has large gaps, line it with burlap, old fabric, or cardboard before adding soil. For the cheapest soil mix, use local topsoil or compost blended with coarse material like coconut coir, sand, or fine bark if you can get it affordably. Avoid using garden soil alone in containers because it becomes heavy and compacted.
Fill the box, plant cheap or free options such as herb cuttings, seeds, divided perennials, or small vegetable seedlings, and cover the soil with fallen leaves or grass clippings as free mulch. Place the planter where it gets suitable light, water when the top inch feels dry, and raise it slightly on bricks or wood scraps so excess water can escape. This repurposed-container method keeps costs very low while still creating a practical urban planter box.