I know there is an easy and not too expensive way to build a platform bed, I just haven't found it. A friend gave me her king size mattress/box spring. This will be for one of our extra upstairs bedrooms. I have 2 pieces of flat quality 3/4 " plywood about 6 x 8 that I keep thinking could be used for this project.
We made one for our son's room. It was a real simple one we saw made on HDTV.
You take landscape timbers, cut 7 legs 2 ft long. Cover them with upholstery fabric and stand them up verticaly and screw the plywood onto them at the corners. The 3 extra legs go in the middle where the 2 pieces of plywod join. You just screw the two pieces onto the tops of the legs to support the middle. Use at least 1/2" plywood. Use 3" wood screws.
We then put a dust ruffle on it, added a matress and were good to go.
The neatest part is that there is no boxed springs, so you have a taller area under the bed that's great for storing all kinds of things. And the bed skirt covers and hides it and the legs.
The legs are the landscape timbers, covered with a neutral apholstery fabric. I just wrapped it around the legs so that the seam was on the inside, and hot glued it on. It was perfect, and cheap for a kids room. We could store most of his ball equipment under it.
my husband actually just did one for my son too. his is 4 sidewalls of mfd put together like a box. then he put a few 2x4 across the inside like ladder rungs and laid the plywood down on top of that. it actually lets the mattress sit about an inch down inside the sidewalls so the mattress doesn't slide around.