I built one for 600ish, but I did get my shingles free from a friend that had extra. I built the floor and walls like a house, except pressure treated for the base. It had to technically be moveable to not require a permit, so instead of poured concrete I used block stacked to make a level base. Tounge and groove plywood flooring sheets. T1-11 sheets on outside. I made the entire end a split door, open half for normal use, both doors to open the entire end wall.
Just design it for economy of materials-avoid leftovers, but you said 12x8, so thats easy.
Its held up 7 years so far,
Brian