Have you considered using gold other than 99.9 purity? There's upsides and downsides to using less pure gold, the upside being it's easier to drum up interest due to the cheaper price (and it helps you as you need less capital to produce your initial run), the downside being that people might not be as allured by the coins if they aren't completely pure.
Yeah, 3rd point above was about looking at mixing another alloy in to keep diameter, and it would also lessen the cost.
But BitcoinNewsMagazine is right, if you start mixing in and taking away the purity, it's not the same to collectors.