I want to cover a small area to be usable in the winter / rain. I am thinking of using some transparent or semi/transparent material to allow light to pass and to be discreet. I found Plexiglas, but I still think it's a bit expensive per square/mt for the work I want to do. I am not even really sure that will work the way I want to. Is there a cheaper alternative?
There are all sorts of translucent roofing products, mostly corrugated. Some are cheap, others ridiculously expensive. Any large building supply company should have the stuff.
Check with both a plastics and a a greenhouse supplier - coroplast or twinwall polycarbonate (different products, both resembling corrugated cardboard but made from plastic.)
However, if you really want inexpensive, design your roof to be covered with flexible green house plastic film (it lasts much longer in the sun than typical builders vapor barrier plastic) as that is pretty close to being the cheapest way you can cover ground.
Home improvement stores will carry corrugated clear sheets with UV protection on one side. They come in 2x12' sheets, in cheap brittle PVC or expensive unbreakable Polycarbonate.(plexiglas)
All PVC types are brittle and wont last long but only $20-25 or so.
The Polycarbonate will last at least 10yrs before getting dull and you can drive your car over the stuff without damage but cost $40-50. or $1/sq.ft. Then add $2/sqft for framing costs every 24x16" I made a Pergola 11x12' x 8-10'h slope on two deep posts. I'd be delighted to share details . This was a huge patio and I made removable walls of same in panels for winter screwed to 1x2 horizontal strips of wood , removed in Spring. I design to carry 6ft of snow or 3 tons into 2 posts with 12" poured concrete footers and 8" sonotubes filled with pressure treated posts surrounded by concrete to surface at 6' -8' deep instead of 3 posts 4' deep due to clay compression is better at this depth here. 3 yrs later it still,perfect level.
Plexiglass does not have UV blocker for skin, which I wanted. But looks smooth and although thicker needs sturdy framing in alum, steel or wood to prevent sagging.