If you are a beginner city gardener, the most affordable urban planter box is usually a basic rectangular plastic planter. Look for one that is 18 to 24 inches long and 8 to 10 inches deep, with pre-drilled drainage holes or holes you can add easily. At discount stores, dollar stores, Walmart, IKEA, or Amazon, these often cost $6 to $15. They are lightweight, easy to move on a balcony, and large enough for herbs, lettuce, radishes, peppers, or a compact tomato plant.
Fabric grow bags are even cheaper—often $2 to $5 each—and they work beautifully in small city spaces. They are not rigid boxes, but they drain well, fold flat when empty, and prevent root circling. If your main goal is the lowest possible cost, a 5-gallon fabric grow bag or a food-grade 5-gallon bucket with drainage holes is hard to beat. A storage tote can also become a DIY planter box for around $5 to $10, but you must drill drainage holes.
For a true planter box, skip trendy self-watering systems, cedar, and galvanized steel at first. They look great, but they can cost $30 to $100 or more. For a beginner balcony planter, a simple plastic window box or railing planter is a better budget choice. Choose a neutral color, check that it has a saucer or drainage tray if needed, and make sure it is UV-resistant so it does not crack after one summer.
What matters more than the container is what you put in it. Use fresh potting mix, not heavy garden soil. Place your planter where it gets at least 4 to 6 hours of sun for vegetables and herbs. Water when the top inch of soil feels dry, and feed with a simple liquid fertilizer every two to four weeks. Start with easy crops like basil, mint, lettuce, chives, radishes, or bush beans.
So, the short answer: for most beginner city gardeners, the most affordable urban planter box is a basic plastic rectangular planter, around $6 to $15. If you are willing to use a soft-sided container, fabric grow bags are the absolute cheapest and most flexible option. Buy one or two, learn how they drain and dry out, and upgrade later once you know what you love to grow. This is budget gardening at its best.