My dad did one for me as a kid that was a simpler version of JustDressageIt's. It was about half the size width-wise and about the same height and depth. He used regular sheetboard and 2x4s (like you get at lowes) and I don't think he stained it. It held up great. Actually, I was recently at my old barn and it is still there. Since it isn't being used, I am thinking of asking the BO if I can take it.
Mine had a full shelf up top and a half shelf about 1-1.5 ft off the floor, a saddle rack and 2 or 3 hooks on the inside. If you don't need the saddle rack, but want a clothes place, you could use that room to make a rack for hangers or add additional shelves for blankets/supplies etc. Or you could make it 4 ft high instead of 6 or 7 ft high and that would make it cheaper and give it a smaller footprint if space is an issue.
I asked my dad recently what he thought it would cost to build it again and he guessed probably about $200 or so, depending on materials. I think I looked at some outdoor sheetboard recently and it was about $30 or $40 for a 4x8 ft piece (about half that if you get indoor sheetboard, but won't hold up as well) and 2x4s are pretty cheap too.
I like JustDressage's idea of adding the plastic storage box. Gives you additional storage without the cost of building drawers. Very clever. :)