Exchange value is not exchangeable with or reducible to prices. Exchange value is the one of the forms under which the class relations of capitalism become visible. I don't see how the "economist" or "economics" can make any particular use whatever of "exchange value," since the term presupposes a historical perspective on social relations.
"Prices" are not the value of anything, for again "value" is a concept by which certain social relations of struggle become available for analysis. That is, it is inseparable from the earlier Marxian epistemological premise that the point is not to interpret but to change the world; that is, the world becomes knowable (interpretable) only within the context of action attempting to change it. Prices have little if anything to do with this.
Carrol