If you are looking for a planter box that is both cheap and easy to build, then you might want to consider building your own planter box. There are many different types of planter boxes that can be built with different materials.
Some people choose to use pallets, which are cheaper than most other materials, but they can also be difficult to find and difficult to work with. Plywood is another option for building a planter box. It's easier to find and it's usually less expensive than other options.
You can also use cinder blocks or bricks as the foundation for your planter box, but these materials are often more expensive than plywood or pallets.