I have always wanted a real greehouse, but my home has a yard that is constantly in deep shade all day. The rest of the time it is under a canopy of huge oak trees -- (all of them in the neighbor's yard so I cant cut them down!)
In October at Loew's I saw a greenhouse kit that was only $39; so I figured "why not." It came with several aluminum poles that fit together, four heavy-duty mesh shelves, and a fitted plastic cover -- flexible with zippered front. It also has wheels so it is easy to move around. I put it on my small deck just outside my office window. Inside I put a large (12-inch diameter) thermometer and at the local boating store (West Marine) I bought a ceramic boat heater.
Bottom line: what an excellent investment! The temperature here in NJ has been in the teens on a few nights but the heater keeps the plastic greenhouse a constant 70 degrees. On sunny days when the outside temperature is above 40, I open the zippers so heat does not build up. (At one point on a warm day (45 degrees) I looked out the window at the large thermometer and it read 100 degrees!)
I have put gernaiums and vinca inside and they are blooming all over the place. Also I rooted some coleus cuttings and planted a few herbs. The seeds I planted are doing well as seedlings, and next I am going to early start caladiums.
For $39 this little project has given me hours of enjoyment. (Oh yes, I am in a wheelchair and this makes my gardening a lot more convenient -- easy access and no bending over!)