SALE PRICE = (FACE VALUE) + (NUMISMATIC VALUE) + (SELLER'S PREMIUM)
Notice the lack of multiplication symbols; statements such as "2x face" or "10x face" have no meaning and no place in numismatics.
The FACE VALUE for a Casascius coin would be the value of the BTC stored within, plus the spot value of any precious metals.
The NUMISMATIC VALUE is an absolute value based solely on the desirability of the coin. This generally correlates with mintage, age, and condition, but other factors can be at play here as well.
The SELLER'S PREMIUM varies from seller to seller, based on a number of factors including the seller's trustworthiness, care in packaging, supporting documentation or certifications, and other factors.
The biggest point to make here is that with all other things being equal, FACE VALUE and NUMISMATIC VALUE have no bearing on one another.
I disagree with Nubbins on that point (we argued over it IIRC). Physical Bitcoins are not your typical numismatic coins and therefore have different premiums. We have a small specialized market with a unique method of pricing them. Face values on regular coins make no difference in prices, but with Bitcoin it makes all of the difference.
I've also noticed that some specific sellers are able to command a higher price than others for similar coins consistently.
I honestly think it depends on the coin. Lower face value ones seem to go from the face x whatever value and the larger coins get a random premium. What sellers get more from sales? It does make sense paying extra to deal with someone you know will deliver with no issues.
As I mentioned before, 25 BTC cas coins tend to sell in the 27-28 BTC range (a premium for 2-3 BTC) while a silver single cas will sell for about the same premium (both as of about a year or so ago).