Mom bought me a greenhouse about 15 years ago when I bought my farm. 12x10x8, double door, two vents; it really rocks. It was a floor sample at a lumberyard that was going out of business. It was $3K, she got it for $300 AND they delivered it! Deal of the century. I laid the brick floor myself, but it needs a little work; a few heaved bricks due to gophers...
I use it to start and harden off veggies, and also to extend the season with salad greens and stuff if I need to. There have been years when I’ve had fresh tomatoes, peppers, salad greens, etc. up until Thanksgiving. But, it is useless in the heat of the summer; unless I wanted to grow agave for tequilla, nothing can survive those 100 degree days in there. :)
In winter I give it up and grow greens indoors for the winter (55-degree fieldstone basement, grow-lights, etc.) because shoveling a path out to the greenhouse under 100” of snow is just not a ‘priority’ for me! :)
$300 is practically free for something like that. I am reading the passive solar greenhouse book (like all good energy conscious books was written in the 70’s) and will build a passive solar greenhouse someday.
In the summer when the temperatures are extreme..you can use it to sterilize your potting mix for the next year.