Harbor Freight has reduced the price of their 10' x 12' greenhouse kit to $599.99 - Both Suze and I bought one last fall when they were $799.99. The assembly isn't for the timid and it's needs to be a bit reinforced, but you definitely can't beat the price - especially if you watch the newspaper for one of their 20% off any item coupons!
You can also find their 20% off coupons in the back of a lot of DIY magazines, such as Home Handyman and Popular Mechanics.
Below is a link to their website and assembly information in PDF format. They also have many retail locations and you'll save the shipping cost if you can find one near you.
Somewhere I found a thread outlining how you assembled your greenhouse and problems you ran into, and now I can't find that discussion. Can you tell me where it was? I was considering purchasing one but was concerened about our New Mexcio sun and wind. I remember reading something about the poly panels not holding up in the sunbelt, has anyone found this to be true?