This greenhouse was built in Vancouver, Canada for less than $200 using new store-bought materials. Thus far (only a few months) it has survived one winter with moderate wind, rain, and a couple of centimetres of snow.

It is built from polythene sheet stapled over a frame of 1"x2" fir, 8 foot lengths of which cost about $1.50 in 2016.

Materials:

About 44 8ft lengths of nominal 1"x2" fir (it's actually smaller, planed down from 1x2 rough lumber)
Six 8ft lengths of 1"x4" pressure-treated spruce
One 8ft length of 1" x 3" fir.
Three 8ft lengths of 1"x1" fir (might be more expensive than the 1x2, which would probably work too)
Many dozens of #10 2"" wood screws (or thereabouts; I used Robertsons)
Dozens of #10 1" wood screws. A dozen or so #10 3" screws.
Many dozens of staples (for a staple gun)
Five 10ft x 25ft sheets of "heavy duty" polyethylene sheet. I think it's about 60grams/square metre - tougher than a plastic bag, but sold in rolls.
A few metres of electrical tape
Carpenter's glue
Two metal hinges
A metal bolt (for the door)
Four corner brackets (for the door)
A dozen or so 3" #8 woodscrews
8 nailing plates
One 4ft x 8ft sheet of 3/8" OSB (oriented strand board, cheaper than plywood)

Tools:

Staple gun
Variable-speed cordless drill (a hand drill and screwdriver would work)
Table saw (a mitre box and tenon saw would work) - about 100 cuts in the 1x2
Hammer (for the nailing plates)
Tape measure (16ft or so)

Step 1: Building the Base

The base is made from 1x4 pressure-treated spruce, wrapped in poly sheet and laid directly on the ground. The ground has to be fairly flat and level. At least, the four corners have to be co-planar.

The base is made from two 8ft lengths for the front and back walls, plus two 6ft lengths for the side walls and another two 6ft lengths which form supports for the planter shelves. See the PDF drawings.

Cut polythene sheet to cover the spruce, with about a 1" overlap. Secure it in place with electrical tape, then staple through some of the tape for extra strength.

Lay out the lengths of spruce on a level surface, and use Pythagoras and the tape measure to true them (3,4,5 triangle). Using a backing plate if on soft ground, secure the corners with nailing plates hammered into the wood, and add some 1" screws for good measure. Add the cross-members evenly spaced and secure with nailing plates.

greenhouse.pdf
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Step 2: Building the Walls and Roof

Lay out the lengths of fir on a flat level surface. Set the fir on edge (the walls will be 2" thick, not 1"). Cut to length and secure the corners with glue and 2" screws. See the PDF file for dimensions (the dimensions are not particularly critical, except that all the parts should eventually fit together). It is important to drill shank holes and pilot holes for the screws, else the wood will split.

Roughly true the frame and add the full-height cross-members. Then true the frame properly to right-angles using Pythagoras and a tape-measure, and cut and fit the cross-bracing elements. The structure gets its strength (shear resistance) from triangular elements. Add at least one diagonal element in each direction, so that one will be in compression with shear loads from either direction.

When the sides are complete, wrap in poly sheet. Make an overlap of a couple of inches, tension the sheet by hand so that it's flat and smooth, and secure with electrical tape. Fasten the sheet at intervals with staples, using small lengths of tape as washers to spread the load. If it is necessary to overlap two sheets of poly, try to arrange that the upper and outer layer is on top of the lower one so that rain will not be funnelled inside the walls in the finished construction. The overlap joints should be made on what will become the inside of the wall.

The door is made in the same way. The diagonal brace should be in compression to take the weight of the door itself, and fit properly (with no gaps), else the door will sag and jam in the frame. I added metal brackets to reinforce the door corners as they will experience more movement (the glue will probably crack). Fit the door to the end wall with hinges and fit the bolt - make sure it is closed to keep the door in position when the wall is erected, as if it swings the wrong way it will break the hinges.

The roof is constructed in the same manner. The lighter 1x1 elements are flush with the upper surface. The diagonal members are not needed when the roof is in position, but are needed to give it some shear resistance while it is being erected.

Step 3: Erecting the Walls

If you have people to help, you can have someone hold the walls in place while you secure them. Else, make a diagonal brace using an 8ft length of fir. Temporarily screw one end to the wall, and fasten the other to something heavy enough to prevent the wall tipping. With two adjacent walls in place, screw them together at the corner with 3" screws. Clearance and pilot holes should be drilled in the fir to prevent it splitting. Fasten the walls to the base with 2" or 1.5" screws.

When the walls are complete, fit the 1x3" length of fir lengthways along the centre to support the middle of the roof. The roof then goes on top of the walls, secured with 3" screws.

A piece of 1x1 fir makes a doorknob to allow the door to be pulled open.

Step 4: Fitting the Shelving

The planter shelves are made from a 4ft x 8ft sheet of OSB, cut lengthways to give two 2ft wide shelves, and covered in poly sheet for water resistance.

Screw lengths of 1"x2" fir to the front and back (long) walls, at an appropriate height (about 40"). Add legs to support the front edge of the shelves. The legs are made from 1x2 fir, resting on the 1x3 spruce base members and secured with screws. With the shelves in position screwed to the supports, the structure is sufficiently rigid that no additional cross-bracing is required.

In the photograph, as I recall the sheet of OSB was shorter than nominal and I had not noticed when I bought it, which is why there's a gap at the end.