Howdy,
I have one of these:
http://www.harborfreight.com/cpi/ctaf/disp...temnumber=42211For $170, I couldn't beat it. Its held up well through at least one (maybe two?) Ohio winters as well. You can also get replacement covers.
Its not a sealed structure... The floor is whatever ground you put it on (in my case, pavement) and the sides have maybe an inch or two gap. I didn't try to seal it as I thought with temperature swings & such that that would trap moisture and be worse than just letting air flow through it to keep the humidity at whatever ambient was.
I've got some bare metal in there and it certainly does surface rust, and does it more than I'd expect it to in a more climate controlled area, but it hasn't been particularly bad. I wouldn't worry about any metal that had any type of coating (paint, powder coat, primer, oil film, etc.).
I've been happy with mine.
Mark