How secure does it need to be? I have built a simple frame using some doubled up 2x4's in the ground as posts, plywood roof and blue poly tarps for side walls and roofing. IIRC most of the lumber came out of construction dumpsters.

Take a look at the ready built sheds at your local home center, they are very easy to build and can be made as simple or elaborate as you want. If you have the room and codes allow you can buy construction trailers or shipping containers for as little as $500.

Aaron