Those aren't comparable costs. The cost given for antimatter is the cost of producing it from nothing. The cost given for gold is the market price of buying gold that already exists.

Consider the cost of producing one kilogram of gold from nothing.

(Consider also the cost of ownership. Gold has a higher-than-average cost of ownership; you have to provide security or it will be stolen. Antimatter's cost of ownership is far, far beyond that.)

We can't afford to blow up ourselves that way.

There are plenty of other ways we can afford, so antimatter isn't top of anyone's worries.