The nice thing about a greenhouse is the environmental control you have in one. Cold days become warm days wet days dry hot days hotter and a longer larger harvests off the plants . Many people get two crops a year by being able to start early and finish late.
Amazon has some $300 10 x 20 kits but Im very suspect of the quality
A better way to go is using galvanized fence posts and a hand bender and if your design calls for it the assorted joints for them.
A bending jig is easy enough to make or to save error a person could buy a jig like this >
High Tunnel