is there a price per square foot that i can figure on when having a carport built? i'm wanting a place to park a boat, truck, tractor, and throw some junk in. i'm thinking about 24' deep, 30' wide, and about 10 tall in the front. just 3 sides and a roof----no slab. i know when building a house theres an average price per square foot, but how bout for this?