Outdoor 10'x6 6'x6 6' Green House Grow Hut w Shelves | eBay
Here's the link to the exact model that I purchased from eBay. The greenhouse itself costed only $85, but the accessories make it cost more. I have already tarped over most of it, but there are so many ways that I can insulate and heat it up.
I believe that good insulation along with a large thermal mass alone can create a +25 to +30F differential from outside temperatures. Even though my paraffin lamp is sufficient, I still bought a 400 watt metal halide light because I want to see if it can keep the plants growing through the winter.
So far the compost pile inside my greenhouse has been working strong for the last week so even with warmer temperatures, there has been around a +11F night time differential which is pretty big when lows have been lingering around 50F. The differentials tend to get bigger the colder it gets as long as the drums of water don't freeze over completely. They usually don't. I used to keep 2 55 gallon drums in there, but this year, I'm using 5 of them with even more insulation than ever.