Why isn't my quarter worth $2.33?

I have a quarter. It's a 25-cent piece, but it's a 1965 quarter. It was minted in 1965, and it was worth 25 cents in 1965. It's now 2013 (yeah, the Mayan calendar apparently does go on). 25 cents bought a lot more in 1965 than it does today. For example (following are based on various internet sources; you should find similar values if you do your own internet search):

• average new house cost:
• 1965: $21.500
• 2010: $272,900
• a gallon of gas: 
• 1965: 31 cents
• 2013: 3.40 (or so)
• a loaf of bread: 
• 1965: 21 cents
• 2010: $1.88

Let's take a specific example: in 1965, a brand new, base Ford Mustang was (depending on your source) around $2,372 (MSRP), while a 2013 base Ford Mustang carries an MSRP of $22,200. In other words (well, in other numbers, actually), the 2013 Mustang costs roughly 9.36 times as much as a 1965 Mustang. Applying that to our 1965 twenty five cents, the equivalent 2013 value of 25 cents in 1965 is $2.33 (and that's truncating instead of rounding! decimal place #3 is a 9).

So, if I have a quarter that was worth 25 cents in 1965, why isn't it worth $2.33 in 2013?  I think it should be. There should be a formula for calculating the "current value" of prior-year coins for retail purchases. But I guess that would be too hard for most retail shops, huh?

Anyway, I sure could have used that extra value tonight as we took my oldest boy to the ER, where our BCBS/AL insurance requires a $250 copay. Would have been really nice to pay for that with about $26 worth of 1965 quarters! (Of course, I don't have that many 1965 quarters, but I have quite a few quarters of various years, many at least a decade old, and that surely would have added up!)

By the way, based on this site, that 1965 quarter is probably worth a buck or two to a numismatic (coin collector, although the term apparently includes tokens, paper money, and related objects). In excellent condition, it would be worth up to $8.  That $8 is certainly more than the $2.33 in my mathematical formula. However, I'm referring not to collector value, but to actual, face value... the 25 cents is not based on the "equivalent value" of 25 cents of the minting era, but rather the face value of 25 cents. Seems quite a loss to the consumer, or a gain for a bank.