the following is an excerpt from The Best of Ask Ed; Your Marijuana Questions Answered, Ed Rosenthal, page 51:
"Mylar reflects more than 90% of the light, while mirrors reflect 70% or less. Aluminum foil reflects about 75%; flat white paint reflects about 85% and semi-gloss white about 80%.
Clean styrofoam reflects nearly 100%."
I don't know where he gets these figures so it's hard to say how accurate they are.
4'x8' styrofoam sheets can be purchased at Home Depot, Lowes, etc. for about $6 each. It is referred to as rigid insulation. Cheap, easy to work with, and extremely reflective.