Haha. We keep the plants alive through the winter too!
The materials, to this point, have cost a grand total of $0. I work with my uncle as a carpenter and we have a lot of spare wood that his built up in our basement over the years. My uncle gave me all the 2 x 4s because he wanted to get rid of some that he was storing, and we had all the screws and tools. We have the plywood for the shelf, but we're going to buy nice, new plywood to go around the frame.
Skill comes with practice! You'll never know whether or not you can do it, if you never try!