Yes, the basic cost of the hoops and cover don't seem too bad, then you find you need to have doors and louvres. Our 21' x 42' Northern Polytunnel, as well as the crop bars and triangulation bars mentioned above for stability, also has double doors at both ends and 6' double louvres at both ends ie one at ground level which hardly ever gets used, and one above. When we did the latest re-cover last year, my OH rebuilt the doors and louvres at a fraction of the cost from the suppliers, buying the timber locally and the polycarbonate online. We also had to replace the timber base rails. It was nearly as much work as putting up a new tunnel, but should last another 16 years
What would be a quite useful alternative for us would be a second set of doors with mesh covers for the summer, so we could have ventilation without the hens getting in
. You can get a sliding door now which would be better in high winds, but in a sheltered site many people just have roller flaps over the doors. There are some expensive options such as mesh sides, louvres in the roof and so on, so it's just a case of deciding how high spec you want to go.