It's surprising how much it's possible to spend on a platform bed frame, especially considering that most such frames are essentially a slab of wood on top of a wooden box. However, that simple construction also means you can make your own platform bed frame quickly and inexpensively, using materials you either already have or can acquire at your local home-improvement centre.
A simple box frame platform elevates your bed to a convenient height and provides structure beneath your mattress -- the two main purposes of a platform bed frame. You can build the box as you would a bookcase, by screwing together four planks in a rectangle and reinforcing it with "shelves" inside the frame. A piece of plywood or more attractive lumber over the top completes the design.
A rail frame is an even simpler design for a platform bed, although it doesn't elevate the mattress as much as a box frame would. To build a rail frame, lay two wooden beams parallel to each other, about as far apart as your bed is wide. Lay a plywood or other wooden sheet on top of the beams and screw it in place. For wider beds like a king or queen, you may want to include a third rail on centre to prevent sagging. This low-lying design is especially appropriate for a futon mattress, as it mimics the traditional low furniture of Japan.
For a truly cheap and easy bed platform, acquire an appropriately sized plywood sheet and five breeze blocks. Set four blocks in a square with the fifth on centre. Lay the plywood sheet on top of the blocks, and your mattress on top of the sheet. This isn't the most attractive platform bed you can make, but it's the cheapest and the simplest to install.
If you have an existing rail-type bed frame, you can convert it to a platform bed quickly and cheaply. You begin this process by measuring the space between the rails of the bed, then cutting a plywood sheet to match those dimensions. Lay the plywood down as you would the box springs, then set your mattress on top.
One of the chief disadvantages of a platform bed is reduced airflow under the mattress. In humid climates, this is a risk for mould and mildew. If you're worried about air flow, you can alter any of the above designs by using wooden slats -- such as furring strips -- run parallel to each other, instead of a solid piece of lumber. Leave approximately one inch between slats for air flow.