Like Mullet said, cypress will last a long time as long as it is wet. In fact if it is kept wet, it will never rot. If your ground is wet, then Cypress would be ideal, as posts, and pilings, for a floor support off the ground. Just a simple lean to, would work for a wood pile. You could put in the cypress posts, and pilings, and then use the pallets for siding, and roofing. A couple pieces of tin, or aluminum, roofing material, and wallah, done. Or you could split your own cedar shakes, or just use a couple of strips of tar paper, on the top, and sides, to keep the rain out. You will have to spend something, no matter what, how much depends on how much you can do yourself. For materials, check around for a "Resale" store. The Alaskan saw mill works pretty good. My neighbor has one. Pretty simple. But you need a chainsaw. How big of a log you can cut depends on how long your chainsaw bar is. You can whittle down a big log, with a shorter bar, by cutting one side, and turn it an 8th turn, and cutting that, till you have a log beam, that you bar will reach across. Anyway, there are a multitude of options, to choose from, and like they said, if you can, wait till you have the money, to do it right. You can dig down about 8-10 inches or so, to form a rectangle, the width, and length, you want, and put in corner stakes, and then nail, or screw 2X4's to those, for a form, and put a layer of gravel, and then a couple of small but larger stones, spaced about the gravel, and lay a piece of hog wire, or fencing, on them, and pour cement over that, till it fills the hole, and take a 2X4 and saw it back and forth, across the 2X4's and the cement, all the length of the rectangle, and that will get the cement down to the gravel in between it. You can go back and forth from one end to the other. Then you can get some help, to slide the wet 2X4 on it's flat side, to smooth out the cement. Just before it sets fully, you can use a stiff push broom to texture the cement surface, or depending how wide the slab is, you can just use a regular house broom, if don't want a smooth, or slick top. Anyway, take your time, and save some money, and like they said. do it right. As for primitive, primitive is what they had way back when, if they had the materials we have now, they would jump all over it. Good luck, and as usual, we want pictures.
Wayne