The difference in cooling ability between the two metals does not even come close to the difference in price between the two metals. Sure, silver is a slightly better conductor, but only by a slight margin, and the price of silver is nearly 50 times that of copper.
Now, I could see silver-plated copper, possibly. That would work. But more often than not, you will see nickel-plated copper, since it is much cheaper and almost as good. And nickel is less likely to tarnish and less likely to be subject to galvanic corrosion.