I agree; nowhere near enough info. I don't even see where it's attached or free-standing. That size is really too small to function as a garage for anything much bigger than a Smart Car. If it's 12 X 20 on the outside, the inside will be a little over 11 X 19. Most cars today, even small ones, are about 6' wide. Then, you want to be able to open the doors fully on each side. I measured my wife's Olds, and it is 12' wide with both doors open. That's assuming it's parked dead center, every time. It's about 17' long, which only gives you a foot or so wiggle room lengthwise, and you couldn't walk around both ends of the car with the door closed.
Perhaps you just want a workshop or shed, and that's fine. Costs could be just about anything. There are so many factors- site prep alone can get pricey if it's not level. Unless you DIY, I think it would be hard to come in much under $10K.