Twin-walled polycarbonate panels or Solexx panels(merits discussed here:
https://permies.com/t/44837/greenhouses/Solexx-Polycarbonate) would be a couple options. Neither cheap, but cheaper than plexi. Corrugated single-wall polycarbonate panels are the cheapest option I know of, but not as aesthetically pleasing or insulating.
Something to consider is whether the entire roof needs to be transparent. Depending on where you are, and the direction the greenhouse faces, and the slope of the roof... the portion of the greenhouse roof nearest the house might not be necessary to have transparent; much of the year it may not occlude light from reaching the plants. Whether you need the heat gain from that sunlight hitting the wall is another question...
'Theoretically this level of creeping Orwellian dynamics should ramp up our awareness, but what happens instead is that each alert becomes less and less effective because we're incredibly stupid.' - Jerry Holkins
