Where can you buy the cheapest greenhouse?

It depends on what kind you are looking for.

Size will greatly impact the price, so determine what you need in your greenhouse (i.e. what is your climate like? will you need to heat it? are you starting seeds in the spring? Are you overwintering tropicals or orchids? etc. etc.)

Depending on these answers, you will spend more or less for a greenhouse. There are lots of different kinds built with a variety of materials, from plastic to aluminum to wood. Also, the exteriors also can be coated in a variety of materials, from plastic cloth to polycarbonate panels to glass. The cheapest, of course is a plastic framed plastic cloth covered small greenhouse. The most expensive would be a wood-framed, glass model.

In my research, for the money, Harbor Frieght Tools has the cheapest, most sturdy greenhouse. You're still going to spend a couple hundred bucks on a greenhouse, but it is a polycarbonate walled, aluminum structure that will likely hold up to many seasons of use. They also come in a variety of sizes.

If you are just getting a jump on the season or starting seeds, may I recommend cold frames instead? Get an old window and build a bottomless box, hinge the window to the top of it and there ya go... about $20. And has all the capabilities of a greenhouse for that purpose without the expense.

Cheers:

Eric