When you're gardening in a small urban space and need a planter box that won't break the bank, the single most cost-effective material is reclaimed wood—specifically pallet wood. It’s often free, widely available behind grocery stores or hardware stores, and surprisingly durable with a little treatment. Alternatively, cinder blocks are a close second if you want zero woodwork.
Why pallet wood wins for budget urban gardening: First, the cost is basically zero besides your labor. Second, it’s long and wide enough to build boxes that suit balconies, patios, or rooftops. Third, it provides natural insulation for roots in hot city summers. The trick is to look for pallets stamped "HT" (heat treated, not chemical treated), so your vegetables remain safe.
If you can't find pallets, recycled plastic lumber (often from old crates) is highly durable but slightly pricier. A third ultra-cheap option is fabric grow bags—they cost as little as $2 each and need no construction, though they lack the structure of a wooden box.
To keep your project truly tight-budget: Use screws from a dollar store, line the wood interior with landscape fabric to prevent rot, and seal the outside with leftover paint or linseed oil. With an afternoon of work, you can build a 3-foot-long urban planter for under $10 in materials—using pallet wood will save you over 80% compared to cedar or redwood. For a zero-cost alternative, stack cinder blocks into a square and fill directly with soil; they last for years and need no tools. So, for the best balance of cost, ease of construction, and longevity, reclaimed pallet wood is your most cost-effective bet.