I did a little research about gold and silver production cost (total operating cost) based on a few mid-tier miners. Based on the numbers, silver definitely has advantges comparing to gold. The production cost ratio of Au/Ag is about 90 while the price ratio of Au/Ag is only about 50. That implies, in general, the silver miners should perform better than gold miners. Or, it suggests that there is more room for the silver price to rise comparing to gold price.
The production cost in APE's recent feasibility study seems to be conservative. It's the production cost for all the metals, not just Ag. I guess that's why. Still, I would hope it's actually much lower than $8.44/oz once the production starts.