Cheap wall material?

I'm looking for some kind of siding to make walls for my grow box (2x2x6). Plywood is kind of expensive for sheets that big and I don't want to use cardboard or some other really cheap/flimsy stuff. I want sturdiness and cheapness (if possible). I've been thinking about this insulation board that's silver on one side...I'm just not sure on the price or where to get it (lowes?). Anybody got any suggestions? They would be much appreciated.

i say your best bet would just be the plywood man

plywood is to high use the foam with the foil i seen it at home depot for under $10 and others here use it and its cheap get some 1x4 furring strips the cheap grade not the stuff the keep up front that they want a arm and a leg for but it is usually back were the trim is and make a frame and they sell 2x4 pieces of plywood for the door

1" shinny foam board and a roll of tyvek-housewrap tape would b the easy way. do not get the "styrofoam" board,little tinny balls get everwere . they make other stuff thats got a darker material foam. blocks light better.

Yea Ive seen those foam boards at Lowes, Home Depot, etc...

Id recommend making walls out of some thin 1/2" or 1/4" plywood, so you get sturdiness, ability to attach things to it, and the outside will look relatively un-suspicious. Then put that foam on the inside for insulative properties.

the stuff we are talking about isnt sytro foam ito is a expanded foam similar to great stuff
if u go with plywood dont use OSB or MDF without seeing if the offgassing of formaldehyde is harmful to plants in a enclosed sppace

awesome guys....I'll probably go with cheap plywood or that insulation board.