Nor can this high exchange value be put down to their scarcity, because if it was just a matter of scarcity, why would not producers of truffles take advantage of their high exchange value to make high profits, by producing more. Once again, the answer is clear. The producers do not produce more, because the cost of production is high, and so profits are not excessive, despite the high price/exchange value.
The value of truffles is high because they require a lot of labour-time to produce. The high value exists prior to exchange, and independently of it. Exchange simply enables that high value to be manifest as exchange value.
Marx sets out what is wrong with the idea that value only exists as a consequence of commodity production and exchange. If value only exists as a consequence of exchange then, once exchange ceases, value itself disappears. If A's value depends upon it exchanging with B – just as B's value depends on it exchanging with A – then once they have exchanged, their value would have disappeared!
“And in fact each stands on its own, outside Lord Exchange, who only consisted of this change of place. B+C+D is now things, not values.” (p 207)
But, of course, the reality is that it is not commodity exchange that creates value, which must exist independently of it. A product has value whether it is a commodity or not, whether it is exchanged or not. It has value, because it is the product of labour. All that commodity exchange does, as Marx says in Capital I, is to stamp this value on its face, in the shape of its exchange value.
“Or exchange “fixes and determines” in the literal meaning of the word. A dynamometer determines and fixes the degree of strength of my muscles, but it does not make it. In this case value is not produced by exchange.” (p 207)