You may save a bit by building your own roof if you can find some cheap sheet metal and have a metal brake to bend it so it has some strengthening creases to prevent vibration and sound amplification, but as for windshields, I would stay away from the plexiglass (factory or otherwise). They scratch up so easily that in a couple of years you will be replacing it if you want to see thru it. The factory glass is the only way to go on this and it is going to run you more than $800 but barring a rock slung thru it from a bushhog or something, it will last forever with no crazing or scratching. My brother in laws Gator has plexi windshield and it is absolutely dangerous to drive into the sunlight as you cant see a thing thru the scratches and it is only about 3 years old.
I suspect that if you have to pay going rates for the metal for the roof, you will end up spending as much or more for the material as the factory roof costs. You can go with plastic factory roof for cheaper cost rather than metal. I have the plastic roof on my 2006 model 900 and it still looks new although I do keep it inside when not using it. We had a bunch of the AC cab model 1100 in Nigeria and after about 3 years the paint was faded badly on all the metal cabs but they never saw a cover.
Good luck in finding some cheap material if you elect to build it yourself. Be sure to put some rubber insulation washers under the metal roof if you use that and maybe use some stick on sound insulation to absorb the vibration noise that will come with the metal skin.
