Make sure you're allowed to build whatever it is you're thinking of.

Permits will add to the cost
You'll need power - how much to run that in and install lights, outlets, etc?
Do you want "water" now or in the future - maybe a crapper. Plumbing needs to be considered before pouring concrete
Want it warm - in your area you want insulation UNDER the concrete.
Want a hoist in the future - you need to pour the thicker pads now.
What about heat?

Slapping up four walls and a roof is no big deal - its all the other stuff that, if you don't plan and invest in it now will just cost you double and piss you off in the future.