There is a great variety of greenhouses ranging from self made to pre-made models.
When interested in buying a greenhouse, it is always best to ask your local store on advice, Google greenhouse builders in your region and consider the size needed.
A self made greenhouse like this one is easy and cheap to make when only wanting a small area for plants.
Pre-planned/made greenhouses such as this one are obviously going to have more benefits to the plants but in the long run, you can only afford whatever you can afford.
They come in different shapes or sizes so remember to do your research!
Metal framed glasshouses are also a great, sturdy option.