You should be able to find lexan (polycarbonate) for under $200 for a 4x8x1/4 sheet. Here it is for $130
http://www.sdbodyline.com/aluninum.html
I wouldn't use Plexiglass it shatters easily.
You can get lexan to crack, but not shatter. It is hard to even crack it. Remember it is bullet proof. Also the colder plastics are the easier they are to crack.