The price of the mineral defines the economically viable mineral deposits. At $1500, a mineral deposit with about 0.3 grams per ton (0.01 ounce per ton) is economical to process. You must process 100 tons to get 1 ounce but the cost of the ounce is less than the price you sell at. If you spend $10 per ton to process that ore, it costs you $1000, and you sell at $1500. Mining is very much about knowing the processing cos for a particular ore, the grade of the ore, and the accessibility of the ore.
It is also true that many mines tend to produce more than one metal. For instance the Kennecott mine (considered a LARGE copper mine) produces annually approximately 300,000 tons of copper, 500,000 ounces of gold, 4 million ounces of silver, 30 million pounds of molybdenum, and about 1 million tons of sulphuric acid. The cost per mineral is often a matter of business choice. If they want to present the mine as a copper mine, they can take all the expenses, take the revenue for the other minerals as expense offsets, and then calculate the cash cost of copper. The same amount of overburden is removed, and the same amount of ore is processed for he entire set of minerals, just the accountants assign costs ... an ounce of gold is about $1200 and a ton of copper is about $8000. It really is a game assigning the cost of production to individual minerals.
Mining companies also can manage the ore they process to match the price of the mineral. If you have a well drilled pore deposit, and know the gradient of mineral deposition ... say it is 0.3 gram per ton on the east side and 0.6 gram per ton on the west side ... you can mix ore or move to better ore as the price changes, as a mine plan.
Short answer: silver costs less because the sale price is lower. That means that economic silver deposits must be higher in silver content than economic gold deposits. The old rule of thumb was that processing cost about $10 per ton of ore.
I suspect that the short answer is that silver is much more plentiful and easier to get than gold.
There is a nice article on seeking alpha about the cost of mining gold. The short of it is that mining gold costs around $700-800/oz, but when you add taxes and royalty you get to $1100-1200. For silver the total cost may be around $20-25. Obviously a complicating factor is that mines produce multiple minerals and at the end they can manipulate the way they express the cost. Many miners have a large burden of debt they have to service and this adds more uncertainty. It is impossible for me to put all this in a simple equation. My best guess is that at $1200 gold is at a bottom Sure manipulation, market swings can bring it lower, but this would make it oversold and an obvious long term investment.
If you just had the main 2 from Kennecott: approximately 300,000 tons of copper, 500,000 ounces of gold, you could account it (completely made up numbers):
Costs of production: $600 M
Gold sales: $600 M
Net cash cost of Copper production: $0
By that accounting the Gold had a $1200 cost of production and a $1200 sale price ... just a byproduct of "free" copper production.
I used to own a bit of Northgate Minerals and they had copper/gold ore. But they accounted everything as part of the net cost of production of gold. That led to a very low "cost" for the gold. The reality is that the costs bandied about are often inaccurate due to the mixed metal content of many/most mines and the variations in accounting.
But if you use the $10 per ton rule of thumb, you need an half an ounce of silver per ton of ore, or about 0.01 ounce per ton of gold. But if you have 5 pounds of copper per ton (0.25%), you are going to sell that for $4 per pound also.
I'm sure my $10 rule of thumb (for open pit) is WAY out of date, but is such a nice round number that I continue to use it. With fuel/energy/chemicals and mining equipment so expensive now, I bet the costs are much higher. If anyone knows the current rule of thumb number ... please share it.