Karen,
Think "Hoop House" not greenhouse. Kind of like a big cold frame with a door. Mine is about 10' x 16', made with 1" PVC pipe and only cost about $400 to make.
Obviously it's midwinter and everyone is up to their eyeballs in snow. I have had hydroponic lettuce, spinach, and Bac Choi going all Winter long and have more than we can eat from just 4 square feet. Our Broccoli and Cauliflower are about 18" high. They are ready to produce as soon as we start getting more sun. Have raddishes, beets, and onions. The hoop house and cheesemaking make Winter a little more tolerable.