so how is the value determined on a 90% Silver Coin that has no other value other then the silver content ?
Multiply spot value X .7234 (or .715 if you are a pessimist) and you will get the face value cost of $1.00 worth of junk 90%. IE, Spot is $16.79 X .7234=$12.14 Face value.
Silver and gold are (relatively) soft metals and, over time, silver content is lost in circulation. The 10% copper content is there because copper is a harder metal and thus less prone to wear and circulation loss. I have two circ rolls I like to use to illustrate the concept. One roll is AU (unfortunately, I have no Au rolls) and the other roll is "average circulated". The weight of the "average" roll is about 93 percent of the weight of the AU roll.
There is not a single answer to your question. If buying in small quantities such as a roll it is not usual to discount the price. Just take the nominal weight and multiply by the spot price. But when buying by the bag, $1000 face value, there is a standard amount by which it is generally discounted to allow for wear. I forget the actually numbers (it is too early and I am still on my first coffee) but they are easily found.
.715 is the industry standard mulitplier to take wear into account. Some dealers go by it some use .7234.