The demand price of various 'informational' (i.e., symbol- manipulating rather than matter-transforming or matter-transporting) instruments or services depends on the expected payoff they can earn for their users. Certain statistical characteristics such as Blackwell's 'informativeness' of inquiries and Shannon's 'capacity' of a transmission channel are examined as to their effect on the payoff to a given user; and the special character of the underlying assumptions is pointed out. As regards the supply price, and thus the cost, of informational services (i.e., of inquiry, coding, transmitting, deciding), a sketch is made of the current incomplete knowledge of comparative advantages of men vs. machines with respect to particular kinds of services; and of certain economies of mass production. An understanding of the factors determining the demand, the supply, and the market prices of informational services is needed for any discussion of the welfare economics of information, and thus of economic policies relevant to education and research.