I bought an 18x20 foot one from a hardware store that was going out of business. It was their display model and I got it for about $ 400. I put a layer of heavy plastic down, then gravel and installed the metal carport on the gravel. I then laid pressure treated wood on the gravel and screwed pine that I had treated with a termite/bug spray and then sprayed an additonal coating of motor oil and diesel fuel on the wood just to be sure the floor didn't taste good to any insects. The ends of the floor are vented to allow air to pass under the floor and keep it from rotting.
I bought additional metal to cover the sides and ends and I don't yet have it all installed. Here is a picture of it a couple of months ago when the fellow (Blaze) was installing my Woodmaster. The vertical boards were orignally 1 inch rough sawn pine that I had....but are not sufficient to keep from bending under the load of the stacked wood and I since have added 2x4 lumber to reinforce them. I believe it will hold around 18-20 chords of wood when filled up to the rafters - I made a small shelf area in the back that I have reserved for kindling and it turns out I need almost none as the fire runs continuously and seldom needs to be re-started.