If you want the cheapest urban planter box that won't fall apart in a year, skip decorative wooden boxes, thin plastic pots, and flimsy fabric bags. The most practical budget answer is a heavy-duty plastic storage tote, usually in the 18- to 27-gallon range, or a food-grade 5-gallon bucket for smaller plants. You can often find these for $8 to $20 at hardware stores, big-box stores, or online.
These containers are usually made from thick HDPE or polypropylene, so they resist rot, rust, and cracking much better than cheap alternatives. A well-chosen plastic tote can last three to five years or more on a balcony, patio, or rooftop, and it gives you a rectangular "box" shape without the cost of custom raised beds.
To turn a storage tote into a durable urban planter, drill six to ten drainage holes in the bottom and a few along the lower sides. Fill it with quality potting mix, not heavy garden soil. If the planter sits on a balcony, place a tray or saucer underneath to catch runoff and protect the surface. In hot climates, choose a light-colored tote so the roots do not overheat.
If you want a more traditional raised bed, concrete blocks are another cheap long-lasting option. They cost about $1.50 to $3 each, never rot, and can be stacked into a simple bed. However, they are heavy and not ideal for renters or small balconies. Galvanized steel raised beds are lighter and very durable, but they usually cost more than plastic totes.
What should you avoid? Thin blow-molded plastic pots crack easily in cold weather or under strong UV. Untreated pine and pallet wood can rot within a year when filled with wet soil. Cheap fabric grow bags may last two or three seasons, but intense sun and frequent watering can wear them out faster. Terracotta and decorative ceramic planters can also crack in freeze-thaw conditions.
For the absolute cheapest urban planter box that won't fall apart in a year, buy a heavy-duty plastic storage tote or a 5-gallon bucket, add drainage, and use good potting mix. If you can spend a little more, a galvanized metal raised bed or concrete block bed will last longer. But for pure budget and reliable short-to-mid-term durability, heavy-duty plastic is the winner.