Patrick,

  Here's my shot at a quick description of "value" in the framework of
neoclassical economics:

1) The the value of an item (say, an extra unit of a good or a service,
whether or not it is sold in an explicit market) is measured by the goods
and services that one is willing to give up to acquire or maintain that
unit.  This "willingness to give up" is typically expressed in monetary
units (say, the dollar value of goods and services that I am willing to
forgo in order to acquire a Hank Williams CD, aka HWCD), but the expression
of willingness to pay in terms of dollars is not of primary importance.
What IS primarily important is the notion that the value of an item is
indicated by what one is willing to give up for it, regardless of whether
this is expressed in dollars or in some other measure (like how many bags of
dog food I'd be willing to give up for that Hank Williams CD, or how many
hair cuts I'd be willing to give up for it or...etc....).  Expressing value
in dollars is not to imply that money is, itself, important.  Expressing
value in dollars is simply a convenience, so that we can avoid the hassle
and confusion of people expressing their values in different ways (Alicia
expressing her value of a HWCD in terms of dog food bags, Bryan expressing
his value of a HWCD in terms of hair cuts....).   As a caution, I'd urge
people not to get caught up with the fact that values are expressed in
dollars, as if money is the important thing.  Money is NOT of primary
importance, but the real goods and services one forgos when making a
decision to acquire a particular item is what matters.  (I know I'm
repeating myself here, but this is a really big deal.)

2) With regard to items used in consumption, one's willingness to forgo
other goods and services to acquire a unit of a particular item (that HWCD)
is influenced by consumer-related stuff.  These are mostly: a) tastes, b)
the availability of other goods and services related to the particular item
[for example, the availability of CD players (a consumption complement here)
and the availability of Hank Williams casette tapes (a consumption
substitute here)]; c) income and/or wealth. It is also influenced by how
much of the item one already has acquired.  See point 3) below.  With regard
to items used in production, the value one places on an extra unit of such
an item depends on producer-related stuff. 

3) One critical distinction is that between marginal value (the value of an
additional unit) and total value (the value of all the units consumed, which
is (roughly) the sum of the marginal values of all of the units consumed.
Typically, the marginal value of an additional unit declines as more units
are consumed.  So I might now own two HWCDs and be willing to give up two
Grapefields dinners ($100) for a third, but if I already owned eight HWCDs,
I'd only be willing to give up a MacD's dinner ($6) for the 9th. 

4) Typically, individual decision makers are confronted with a choice about
whether or not to pursue an action which increases or decreases the quantity
of an item by some bit.  For example, Alicia, who owns two HWCDs has the
opportunity to purchase an additional one for a price of $16.  If she values
that extra unit at $100 (two Grapefields dinners' worth, based upon her
tastes, income and the prices of, say, Hank Williams tapes), she will find
it worthwhile to purchase the third HWCD and will enjoy a surplus of $84
(about one and one-third Grapefields dinners' worth) from the purchase.  In
theory, one purchases additional units as long as the value one places on an
extra unit exceeds the cost one bears (here, $16 or about 1/3 of a
Grapefields dinner) If Alicia's consumer-related factors (tastes, incomes,
etc.) yield a value of a fourth HWCD of only $13, she will not purchase the
fourth HWCD.  Note that value reflects what Alicia is willing to give up for
an extra HWCD, and the $16 price is what she has to give up to acquire an
extra CD.  Alicia is essentially undertaking her own little benefit cost
analysis in deciding on the purchase of the third, and then the fourth HWCD.

5) Typically, government decision makers are confronted with a choice about
whether or not to pursue a policy wich increases or decreases the quantity
of an item by some bit, and there may be no market in which the item is
available for purchase.  For example, the EPA may have to decide whether to
recommend a policy which would reduce concentrations of some pollutant (that
is increasing air quality by some amount).  Units of air quality cannot be
purchased in a market (aside from certain situations), but must be acquired
through regulation of polluters in one way or another.  For any proposed
regulation, the EPA would submit a regulatory impact analysis which includes
a benefit-cost analysis, in which the EPA reports its estimate of the
benefit of the proposed regulation as well as the cost (to polluters and the
government agency) of the regulation proposed.  Conceptually (and ideally),
the estimated benefit of the proposed regulation is the estimated value of
the improved air quality in terms of other goods and services we in society
are willing to forgo in order to acquire that air quality improvement
(expressed in dollars).  A narrow benefit-cost-analysis test would lead to
an acceptance of the proposed regulation if the benefit exceeded the cost of
the change in air quality.  Economists don't necessarily recommend a narrow
benefit-cost-analysis test in public policy decisions, but it does bear a
resemblance to the kind of decision-making described for an individual in
4). 

6) Here's a caution, sometimes the term "value" is used as I have here, to
mean "benefit" to an individual or other maker where the benefit may be
weighed against a cost).  Sometimes the term is used to to mean "net
benefit" which is the benefit minus the cost (in which case, a
decision-maker acquires an extra unit of an item only if the net benefit is
non-negative).  Just look at the context.  But the underlying logic is
unchanged.

--Jan