In the last 25 years my dad has built nearly 1000 buildings like this, the smallest i would go with would be a 30x40, if you don't need to don't go over 10 foot on your side walls that will really help with cost. I can't remember exactly what the last one we bid came out to but i think it was real close to 22,000. That is 10 foot tall with a 16ft insulated door, the best painted metal you can buy, and gravel, concrete, rebar, labor, wireing. That did not cover any insulating or lineing the inside with metal but that can be done later. You could save some with galvalume rather than painted for the roof and walls. I really think you should be able to get a 30x40 fineshed pretty nice for 30,000 or less.
