[cyberax]

Gold is not great for bulk traces, it has worse conductivity (thermal and electric) than copper. It's used for coatings because of its corrosion resistance and its incredible ductility.

Silver would be better than copper, but the difference is not large (~8%) enough to justify the price differential.

[Schlagbohrer]

For those of us who appreciate PCBs as works of art, it never occurred to me that I could make a silver plated board just as a flex.

[rkangel]

I think that in addition to the cost you would also have a corrosion problem. That's usually the reason that silver isn't used as an engineering material.

[cyberax]

You totally can! But it'll just look like it's made out of aluminum :(