We’ve built this greenhouse for about $300 all new material (+P.T. lumber) and a single layer of 6mil plastic. My wife and I built it in 1 day. Add a second layer of 6mil and a 12v blower or some spacers for <$400 total cost.
Valeries linked greenhouse will have more curb appeal though after a season or 2.
We also did this in our big greenhouse over a year ago. We’re happy with the results so far. I would have done a couple things differently in hindsight.
We put the geothermal in and doubled our greenhouse footprint. It cost us about $3k altogether using craigslist for some and buying new for some.
Our winter/starts heating bill went from around $2K+ to around $200. So the payback is very good. But, we did not use extra heat the coldest 2 months of the year as we had previous years. We just kept things from freezing, not heating it to 60. We have citrus/avocado trees that made it through the coldest 2 months last year using only geo and frost cover.
Just one link, but again, you can go as deep as you want.