The short answer is that DIY is often cheaper only for simple, small planter boxes—and only if you already own the tools and can use inexpensive or reclaimed materials. For a basic wooden box on a balcony, building it yourself may cost $25–$60 in lumber, screws, liner, and drainage materials, while a comparable ready-made box might cost $40–$150. That price gap can make DIY attractive, especially if you want a custom size or have leftover cedar, pine, or composite boards.
However, the sticker price of materials is not the full cost. A fair comparison should include tools, time, sealant, sandpaper, brackets, landscape fabric, gravel, soil, and plants. Soil and plants usually cost the same whether you build or buy, so they should not decide the question. Tools can change everything: if you need to buy a saw, drill, clamps, and weatherproof screws, your first DIY planter may cost more than a mid-range store-bought model. Your time also matters. A simple box may take one to three hours; a larger, lined, finished planter can take a weekend. If your free time is limited, buying can be the better value.
Bought planters have hidden value too. They come with consistent dimensions, pre-drilled drainage, a finish, and sometimes a warranty. They are easier to return, and you can inspect the weight and build quality before buying. For renters, lightweight resin or fabric planters may be safer on balconies than heavy DIY wooden boxes. For exposed rooftops or windy sites, a poorly built DIY box may fail sooner, making the cheaper option more expensive over several years.
DIY wins when the design is simple, the size is unusual, and you already have tools and skills. It also wins when you want to reuse pallet wood, cedar offcuts, or other materials and do not count your labor as a cost. Buying wins when you need a durable, weather-resistant, attractive planter quickly, when safety and weight limits matter, or when the total cost of tools and mistakes would exceed the retail price. For most urban gardeners, the cheapest reliable route is to compare total cost per year, not just the first purchase. A $90 bought planter that lasts eight years costs about $11 per year; a $45 DIY box that lasts three years costs $15 per year before your time. That is the number that really answers whether building or buying is cheaper.