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I wouldn't go for a plastic/vinyl shed because they trap condesation inside of them and you may find the things you have inside start getting damp. A quality wooden shed can breath and in my opinion look smuch better. I got one from a company called Tiger Sheds (you can see their site at http://www.tigersheds.com) they're based in the UK but have a reputation for doing really high quality sheds so it would be good to use them in a comparative way maybe.
for $678 I am not sure of the dimensions though.
Costco does have a great one. It's about 8x8 or thereabouts, $700 or so. It's vinyl, comes with the base and also has some shelves inside. Very good quality resin type material so it will never break. Well worth it I think.
but you could check the website