Bear in mind cheap isn't cheap if it splits and breaks at an inopportune moment and you lose your crops.
Also if the light intensity is reduced such that your plants etiolate you will be wasting your time. There is a reason why all the polytunnel companies charge a lot for the polythene and it is not because they all have their heads together. They would seriously undercut the prices of others if they could.
If your polytunnel has metal frames you will also have to fork out for anti hotspot tape as well to prevent the pipes burning the plastic. Initially I was taken aback by the cost of polytunnels but decided not to embark on total DIY for these reasons. At the end of the day its only money.
