how good are you at DIY?
if your at all handy why dont you construct your own, in true STW fashion.
cheap prefabricated sheds are woefully bad! the construction is poor and generally flimsy.
i built a shed/office (about 4m x 4m) using 2nd hand scaffold boards,
£5 per 13ft board,
used them to clad and as the floor/base.
its easy really if you are that way inclined,
the materials will come to the same as a similar sized prefab shed,
but it produces a proper solid structure that wont be blown over,
it was a great project!
otherwise proper sheds are v expensive, your looking at thousands,
depends if your looking for an allotment type shed or to create another room shed
drop me a line if you wanna see some build pictures