The adage is: you get what you pay for. Smooth-On products are worth every penny.
And even if you go with something like Brick in the Yard ( http://www.brickintheyard.com ) you're only going to end up saving about $3 a pint.
TAP's Quik-Cast is actually MORE expensive than Smooth-On per pint: http://www.tapplastics.com/product/mold_making_materials/casting_products/tap_quik_cast/74
Alumilite is the same way: http://www.alumilite.com/store/p/665-AlumiRes-RC-3-Tan.aspx
So, it's not that Smooth-On is expensive, it's just that, that is pretty much what Urethane casting resins cost.
...and if you're going to spend roughly the same amount of money regardless, why not go for the best? Which is Smooth-On in my opinion. I've been to their headquarters/warehouse/production facility and they really are the most professional group you could ask for and will answer ANY questions you have about their products or their applications.
And in the grand scheme of things, if you're making helmets the price of Resin is negligible. (Remember you'll also have to buy silicone too...and all the supplies to make your master) If it's still too much, you may want to examine other methods of creating helmets that are ore cost effective like scratch building, pepakura, or modifying something off the shelf.