You will have to find where you can get the best prices for your material. I buy my plastic from a Greenhouse Wholesaler in Minneapolis, JRJohnson.
Get at least 4 mil, top grade will last 5 years even though they guarantee it 3. You need at least 33 feet of 24 foot plastic, or if you double it, like I do, 65 or 66 feet. Alternatively, you can buy 33 feet of 48 foot plastic. You fold it in half and pull it up and over from one side. I attach temporarily at the ridge on each end to hold it from blowing away and then go to the side toward the wind and attach it there. Then stretch it across to the other side, pulling as tight as you can. Then redo both ends. After it is all assembled, you can inflate it like a balloon by installing a small blower to force air between layers. This is additional cost, however, it helps if you are going to use it in very cold weather.
I use Pressure treated plywood and 2" X 4"s that you can get from the lumberyard. You probably won't buy that as good as I do. It will take 16 sheets, plus whatever you use for doors. Figure 18 and you're covered. Buy 35 2" X 4"s for framing and you will have extra scrap. These side and end walls are an important part of the construction. They add weight so it won't blow away and also rigidity. You can cut some cost by substituting some more plastic for part of the plywood, and it will let in more light, but your structure won't be as good.
The conduit is 1 1/4" EMT for hoops. I pay $6.95 each and a dollar each for bends and flares from my Muffler man. You can use couplings instead of the flares, but it is hard to beat the magic of a 120 degree bend done by a professional. Nine of the conduits are bent in the middle and flared at both ends. These are your ridge pieces. The other 18 are simply bent at the four foot mark so 6 feet goes up the rake of the roof. You assemble all 9 hoops on the ground. Then attach them one at a time to one sidewall, or half of one side wall by drilling a hole through and lagging to the upright 2" X 4" with 1/4" X 2" long lag screws top and bottom. Procure 36 of these at your hardware store. Then when you have 9 hoops standing, and bolted to one complete sidewall, stand the other sidewall and attach it. Then square it all up and stick frame in the end walls.
I attach legthwise Purloins made of 3/4" EMT conduit at the joints between the upper and lower sections of hoop. This adds stability and gives you a place to hang baskets. Drill 1/8" holes and attach with 1 1/4" #10 Sheet Metal Screws.
I buy the cheapest scrap 1X2 I can find for nailing on the plastic. Remember, this will last five years and you can buy it again the next time you recover the house, or use it over, if you wish.
If you want to add some class, order a load of pea rock delivered and spread it out ahead of time on the area you are going to build. Then you can use two foot patio slabs for walkways between tables. This is a beautiful Greenhouse that will stand the toughest storms and even the snowload of a Northern Minnesota winter.
E-mail me if you have any further questions. Also, you might try an alternative design which I have not tried yet, made out of PVC. See plans at: