The cheapest bit of a polytunnel is the hoops - those are the bits offered cheap on fleabay. The most important bit is the instructions, then you need bracers and cross bars and a ridge pole and louvres and doors and fixing bolts and anti-hot-spot-tape and a nice new piece of polythene cover, plus crop bars possibly.
What size are you looking for? What is your local weather - windy or sheltered?
We are windy and our tunnel is 7m x 14m - average for a smallholding. We went to Northern Polytunnels as their hoops are sturdier than many and they offered all the reinforcements our site required, and a choice of covers. All the odds and ends certainly add up, but it's worth getting a good quality tunnel - saving a few quid only to see your tunnel fly away in a gale and your crops lost is just not worth it.