Yeah, if you are on a budget, cnc will not be the way to go unless you slice it as I and Gremlin have mentioned, and use MDF or something. A big block of aluminum is an expensive and unnecessarily heavy option, and even then I doubt you could fit a helmet sized block on a six-axis machine. You're looking at a couple hundred bucks BEFORE molding costs for aluminum big enough for a helmet. The cheapest and best way to use a CNC machine is to build two halves of the piece cut into MDF, and then smooth it with spot putty and bondo, and then vacuum form the halves and stick them together.
But if money is an issue, pepakura will be worlds cheaper since it gives the same result without buying wood or metal, and is wearable right off the bat without a mold or vacuum forming machine.