Your free 1/2 goldback has 0.0005 troy ounces of gold in it. That's just over $2 worth of gold.
Then of course they'll be happy to sell you that denomination for just over $4. Or other denominations for about twice as much as the value of gold in them. For example their 50 goldback "bill" has 0.050 oz of gold in it. That's just over $200 worth of gold that they're selling for $404.
They're essentially betting that at least some of the people that would bother to want this essentially worthless free note would also be kooky enough to think they're ivesting in something or somehow "defying the grid" as they put on their site by buying these other bills no one will know what they are let alone take or value at what you paid for them.