We bought an O'briens shed last summer. 8ft by 12ft I think. A pretty good shed by the looks of it, and if memory serves me it cost the folks in the region of Euro1,200.
The snag is you have to lay out a foundation of blocks to their specification before they'll erect the shed. That, I have to admit, can be difficult enough to get right.....It was a few hours work, and we've done plenty of that kinda stuff before. That said, we were puttin it up on fairly rough ground.
Shedman is probably the same though in terms of preparation. O' Briens had ours up and sealed in 15-20 minutes.