Panda plastic is the cheapest way to get the room up and running. The Mylar is an upgrade not included in the under $100 price, but it generally runs about $1 a foot for a 4 foot wide roll. You'll need about 16ft or so for a 4x4 square room. Or double that if you want to do the walls all the way to the top.

I don't know exactly how much more reflectivity you will get with the Mylar over the Panda plastic, but Mylar supposedly is the most reflective -- so theoretically you could wrap the frame in it (the roof isn't too important as your lights reflect the light downward) -- but if you did, Panda plastic would be redundent.

If I did the frame in Mylar, I'd just get cheap gardening plastic to shroud it and cut out any light leakage. Get thick plastic to keep it nice and dark in there when you want it dark, and leave a big flap overhang for the doorway.

I do believe the plastic shroud maintains the "hothouse" effect better than just Mylar would -- but I have no proof on that theory as I have not built a Mylar room yet.