I've always been surprised at how they build those sheds, and the corners they cut to get them done. If you were to do it yourself, I'm sure you wouldn't use those little nails they love so much, or the siding that isn't sealed, or doors that will fall apart. If a corner can be cut to save a penny, it will be.
If you have the time and ability to build it yourself, and know how to frame properly, you can easily build twice the building for half the price. Hiring it done by a guy like me is where it gets tricky as to what is better. I can build a much better shed, but my wages will cost alot more then the sheds you get already built.
What really drives up the cost when you do it yourself, or hire one built is that most people decide on a concrete floor instead of the plywood and wood floors that you get with those sheds. The expense is significant, but the difference is you get a permanent foudnation that will never rot on you. Besides the sloppy craftmanship, the small nails, the wide spacing of the studs, not using double plates on top of the walls, not using house wrap and quality siding and roofing materials, you also end up with a building that is restion of a floor that is sitting on blocks that will rot on you, move on you and become home to all sorts of bugs and rodents.
For quick and easy, they are OK. For permanent and secure, they are questionable.
I like metal sea containers too. I have one right now that I'm going to turn into a 24x72 storage barn with it being the enclosed, secure part of the barn. Like many of my projects, there is alot of "gonna do" to it before I actually get started. LOL
Good luck,
Eddie