The difference between using, for example the Price to Earnings ratio of a stock and using multipliers to value casascius coins is that a stock's (company's) earnings is what almost always drives the value of a stock, while the loaded amount does not drive the majority of the value of a casascius coin.
The value of a stock is the present value of the underlying company's future cash flows, after taking into account things like risk free interest rates, inflation, and of course the risk associated that certain assumptions may not turn out to be true. Most of the time, a company's earnings growth will also be taken into consideration, and investors will use both a stock's P/E ratio and it's PEG (price to earnings growth) ratio.
Sometimes, there need to be more complex calculations to determine what a stock is worth, for example, when there are rumors of a pending all cash buyout offer, when a company has large amounts of cash on hand, among other reasons. In these scenarios, it would be assumed that in a worse case scenario, the company can (and possibly will) return a portion of that cash back to investors when management determines they cannot effectively and prudently invest excess cash.
Casascius coins on the other hand derive their value very differently. As I am sure you know, casascius coins do not generate any kind of cash. If the same logic was used to value casascius coins as is used to value stocks (and other financial instruments) then the value of casascius (and other physical) coins would be
below their loaded value plus the value of the precious metal contained in the coins. Since the coins generate no revenue, the only future cash flows would be from when the coins would be eventually redeemed, and when the coins are sold from scrap metal, which would be sometime in the future, and taking into consideration that there is a non-zero risk factor in buying the coins, and a risk free interest rate. If this was the case then it wouldn't even make sense for anyone to even create the coins in the first place because it would be a money loosing venture.
As is the case with other collectable items, casascius coins derriere their value from their rarity, their condition and their desirability. All of the above is very difficult to quantify, so someone trading collectable items may not be able to accurately calculate how much a particular coin is worth. One very good "slogan" to go by to value a particular coin, is 'a coin's value is whatever a buyer is willing to pay for it', although this would somewhat ignore my above statements.
One very good example as to why multipliers are not a good indication as to what a coin is worth is a recent sale of a casascius "exception" tenth graded MS67. A casascius coin with it's face value, graded MS67 will generally fetch somewhere in the 1.9 to 2.5 range, which works out to between 19x and 25x the face value of the coin. The coin in question was accidentally loaded with an additional 1.65, meaning it's total loaded value was 1.76 BTC. If you were to use the same multipliers as a "normal" MS67 casascius tenth, then one would presume that such a coin would fetch somewhere between 33 and 43 BTC, however this was not the case and it fetched somewhere in the 3-4 BTC range, which represents approximately the same premium over the face value of the tenth that a normal MS67 tenth would fetch.
Am I willing to admit that I am wrong about this? Yes absolutely, although I don't think your arguments are strong enough to prove me wrong.
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