Hmm..What prices are too high? Is $200 too high? Masks are extremely hard and that's why they demand such a high price. However, making one, if you know how, yourself will be cheaper obviously..
I would use plastic or similar. You can find Sintra (aka PVC Foam) at plastic specialty shops, or even Acrylic sheets. The Sintra should form a lot easier to the curves needed to make a helmet. I think there's even a pepakura file for this helmet since it used to be extremely popular. You could use that and cut the pieces out of the material you are using.
The alts are things like craft foam or cardboard which, when done right, can also bring amazing end results.