The easiest way to fill a large urban planter box without spending a fortune on soil is to stop thinking of it as a giant flowerpot and start treating it like a miniature raised bed. You do not need expensive potting mix all the way to the bottom. Most plants only need the top 6 to 12 inches of good soil for their roots. Everything below that can be cheap, free, or recycled bulky material.
Step 1: Make sure it drains. A large planter should have drainage holes. If it does not, add them or create a drainage layer that lets excess water escape. Do not fill the whole box with heavy, waterlogged soil.
Step 2: Fill the bottom half with free bulky filler. Plain cardboard, brown leaves, straw, untreated wood chips, small branches, pinecones, shredded paper, and dead plant stems are all good options. If you want a slow-release nutrient boost, use the hugelkultur method: put untreated logs and branches at the bottom, then twigs, leaves, grass clippings, and a little compost. The wood rots over time, holds moisture, and feeds the soil. Avoid treated wood, glossy cardboard, diseased plants, invasive weeds, meat, dairy, and pet waste.
Step 3: Add lightweight fill if weight matters. On balconies and roofs, weight is a real issue. You can place upside-down plastic nursery pots, empty plastic bottles with caps, or milk crates in the bottom to take up space. Cover them with landscape fabric or burlap so soil does not wash into the gaps. This is a practical urban hack, though it uses plastic. For a greener version, use organic filler only.
Step 4: Top with a smart soil mix. Save your money for the top layer. A good cheap mix is about 50 to 60 percent topsoil or raised-bed mix, 30 to 40 percent compost, and 10 percent perlite or coarse sand for drainage. You can also mix cheap potting mix with compost and a little garden soil. If you are growing carrots or other long roots, use deeper good soil and less filler. For lettuce, herbs, and flowers, 6 to 8 inches of good soil is often enough.
Step 5: Use free and local sources. Check with your city for municipal compost or mulch. Ask tree services for free wood chips. Use coffee grounds from cafes, leaves from neighbors, grass clippings, and homemade compost. ChipDrop, Freecycle, and Facebook Marketplace can also be useful. Never use soil from a vacant urban lot without testing it for lead and other contaminants.
Step 6: Water, settle, and top off. After filling, water deeply. Organic fillers will settle as they decompose. Add more compost or soil mix to keep the level where you want it. Feed your plants with compost tea or an organic fertilizer because the filler is not a complete soil.
Bottom line: The easiest cheap method is a filler layer of free organic material or lightweight recycled objects, covered with a weed barrier, then 6 to 12 inches of a compost-rich soil mix on top. This gives plants what they need while avoiding the cost of filling the entire planter with bagged soil.