Here’s Rothbard explaining it. It’s from his Chapter on J. B. Say.
Prices of productive factors must be high for a reason; they are not preordained to be
high…
But where do ‘costs’ come from? And why are they somehow fixed,
exogenous to the market system itself? How are they determined?..
On the valuing or pricing of the services of the factors (or as Say would put
it, ‘agents’) of production, Say adopted the proto-Austrian in direct contrast
to the Smith-Ricardo tradition. For since subjective human desire for any
object creates its value, and reflects its utility, productive factors receive
value because of their 'ability to create the utility wherein originates that
desire’.
In other words:
costs are determined by selling
price rather than the other way round…
Ricardo, writes Say, believes 'that the value of products is founded
upon that of productive agency’, i.e. that the value of products is determined
by the value of their productive factors, or their cost of production.
[In other words, the value of a car or refrigerator comes from all the steel put into making it]
In contrast, Say declares, 'the current value of productive exertion is founded upon
the value of an infinity of products compared one with another … which
value is proportionate to the importance of its cooperation in the business of
production …'.
[In English, the value of steel lies in it’s usefulness in making cars and refrigerators]
In contrast to consumer goods, Say points out, the demand
for productive factors does not originate in immediate enjoyment, but rather
in the 'value of the product they are capable of raising, which itself originates
in the utility of that product, or the satisfaction it may be capable of afford-
ing’.
In short, the value of factors is determined by the value of their prod-
ucts, which in turn is conferred by consumer valuations and demands.
The causal chain, for Say as for the later Austrians, is from consumer valuations
to consumer goods prices to the pricing of productive factors (i.e. to costs of
production).
[=the right way of looking at it]
In contrast, the Smithian, and especially the Ricardian, causal
chain is from cost of production, and especially labour cost, to consumer
goods prices.
[= the wrong way]
If you look into that chapter, you will see the policy consequences of the two points of view.
Book is available here for free: http://mises.org/books/histofthought2.pdf
EDIT: Thinking a bit about it, I notice it’s just a restatement of the law of supply and demand. By the universally recognized Law of Supply and Demand, factors of production have their price determined by demand for them. They are in demand to the extent that profits can be made using them to make cars and refrigerators. Those potential profits, in turn, are there only because individual consumers want to buy the cars and refrigerators. Seems inevitable, no?