Cheap greenhouse? Depends what you need. I have one of those very cheap plastic ones (£25?), the plastic has lasted about16 months, now ripped to shreds. (I could have made it last better if I'd been more careful tying it securely at the very outset and protecting corners with bags of compost etc). A new cover will cost me about £15 max and I reckon it's a good investment at that price. Mine is walk in with one shelf down the side. I've now also bought an even cheaper 4 shelf one for about £10-15 (just four shelves in a cover). This time of year is probably when you will get bargains.

They are cheap and nasty products but at the price I think they do a fair job. Certainly I can't afford £150 for a cheap permanent one.