Short answer: the most affordable urban planter box you can buy online is usually a basic rectangular plastic or resin planter. Expect to pay about $15–$30 for a 16–24 inch model from Amazon, Walmart, or Home Depot. These are lightweight, UV-resistant, and easy to move on balconies or patios. Look for drainage holes, a matching saucer, and thick walls; very cheap thin plastic can crack in sun or frost.
If you are open to a flexible container, fabric grow bags are even cheaper—often $5–$15 for a 5–10 gallon bag—and they work well for tomatoes, herbs, and greens. They are not rigid boxes, but they are the lowest-cost way to start an urban garden. For a more decorative option, galvanized metal troughs and basic cedar boxes usually start around $30–$60.
To get the best online deal, compare the total price including shipping, choose local pickup if available, and shop off-season. Search for "affordable rectangular planter box," "plastic balcony planter," or "urban planter box sale." For most people, a simple rectangular plastic planter with drainage is the most affordable true planter box; fabric grow bags are the cheapest overall alternative.