.75mm Makrolon (polycarbonate), which is effectively lexan. Great material, pretty thin. You could order a 2' x 4' sheet at mcmaster for $19:
http://www.mcmaster.com/#standard-polycarbonate-sheets/=mfsec2
You're paying $137 for the laser cutting (and programming). If you don't mind drilling and cutting your own, you could save a lot of money