With what we spend on them anything that can keep the crap of them is well worth it in the long run, plus during the summer (if we get them) keeps the harmfull UV rays of the paint and from attacking the materials through the windows and also the plastics.
I know I done mine with breathable felt etc to stop condensation dripping etc but doing the work yourself its possible to put one up the same for under £500
new 4" steel box section 7.5m lengths 2 off .................£120
15 off 3.9M lenghths of 100x35mm framing timber
30 off m8 by 100mm lighting bolts
2 tubes silicon
20 off 4.2M (14ft) slating lathes .................................£135
18 sheets of heavy gauge roofing panels pre used works out around £75
Roofing felt I had here but around £60 per roll for breathable but would do 2 ports
Nails for air framing gun i had here
I had the steel plates that the uprights were welded to and all the angle brackets and steel plates for hangers allow £75
Thats about it so not that expensive, The steel supports I just mig welded.
All in all pretty reasonable on the material side