Until only recently, a single article could not be purchased by itself. If the trade in knowledge were judged to be important to the economic health of this nation, the Justice Department would ignore Microsoft and investigate, instead, publications with names like The Journal of Eclipse Measurement or the John Donne Review for their monopoly on information. Moreover, no one has precise data on who reads articles. We know who buys journal subscriptions, and some studies have shown who takes them off the shelves in libraries, but we don't measure usage at the article level. Even citation studies, which give us some indication of use and value, have their critics.
Price is, of course, no measure of quality or value.
The costs of production, from editing to printing, can be determined, but the price of articles or journals varies more widely than those costs. Studies that account for the numbers of characters, illustrations, and equations show no correlation between those variables and prices. Pricing seems to be what the market will bear, and the market in scientific, technical, and medical articles accepts a much higher price than the market for articles in the humanities. Precision could be brought to a system that is collapsing of its own weight. The authors who regularly contribute to a specific journal create the value of the entire bundle of articles in a journal.