How should marginal costs be considered when determining prices?

The basic rule of setting prices is that a firm must
produce a quantity of goods or services that allows the marginal revenue to be equal to
the marginal cost.  This is called the profit-maximizing point.  From this point of
view, a firm should determine the optimal quantity of products to make and should then
set its price so as to sell all of those products.

However,
there are cases in which firms should vary their prices.  There are instances in which a
firm should set prices for some people higher than marginal costs and sell to other
people at a price that is lower than marginal costs.  This is called price
discrimination and should be used when it is not possible for someone to buy at a lower
price and then turn around and resell the thing they have
bought.

For example, let us say that an airline has empty
seats on a given flight on the day that the flight is scheduled to leave.  The empty
seat will cost the airline money.  Therefore, the airline should sell that seat cheaply
to anyone willing to buy it so long as the price is higher than the marginal cost of
carrying that passenger.