We built a greenhouse when we first moved onto our acreage. Its not much, just a pressure treated base at 14ft by 10ft with a framed front door, rear part wall and a pvc hoop for the rest. It was over 10 years ago now when we did it but it only cost a few hundred dollars and lasted about five years before I had repairs to do. Right now its in pretty bad shape and in need of a redo. I will be doing it in the next little bit and will have pictures and video of the redo.
For now here is a Dream Greenhouse I drew up, one day I hope.