How do you build an affordable greenhouse?

You can build a greenhouse with virtually no expense whatsoever if you're willing to do some scrounging or salvaging, and if appearance is not a high priority. A greenhouse is nothing more than a structure which lets sunlight in to growing plants while keeping out cold temperatures. Home greenhouses are frequently built with salvage lumber or metal for a frame; it only needs to be able to hold itself up against wind and its own weight. It can be covered, or "glazed", with salvaged Windows or glass doors, or with clear sheet plastic. Just remember -- the less expensive the glazing, the more repairs it may need to keep out the cold. Finally, you need to find a way to vent the greenhouse if temperatures begin rising too high. It can be a fan of some sort, or even just a simple door that can be opened. If you are in some of the colder climates and wish to grow in the middle of winter, you will need to find a way to heat the greenhouse, but even a simple woodstove or tent-stove will do the job if you're willing to keep filling it. Don't forget to make some way to water the plants; they no longer have any access to natural precipitation.

All other components (fans, shadecloths, shutters, auto-vents, auto-irrigation, etc.) of a greenhouse are for the grower's convenience in maintenance. It only matters in terms of how much time you're willing to spend with your plants.