I generally agree with the math however, if the audit included a core sampling to ensure that these aren't gold plated tungsten, you could expect the cost to increase substantially.
That said, a properly conducted random sample of about 600 bars with core sampling (all of which were authetic gold) would not cost that much more than what you suggest.
In any case bailing out the banks with the first tarp money of 600 bil was 40 billion times more potentially costly then the said audit at 15 million.
Even at the potentially excessive 15 mil, knowing your wealth exists is just a matter of perspective. The value of the bars at 1500 per ounce and standard 400 oz bar is 420 bil.
The cost of such an audit at 15 mil is .35% of the value of the gold. A decent return if there is a scam and not unreasonable to pay every 10 years.
orgy