You don't price the coins you buy/sell in terms of their premium, but rather their absolute value?
This is as good a time as any to remind people how numismatics are priced.
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.
