How can we put the idea of "exploitation of knowledge" on a theoretical footing akin to the theorization of exploitation of labor by Marx? In Marx, exploitation is tied up with the notion of surplus and under capitalism specifically to the concept of valorization, the creation of value by labor; more value than is needed to reproduce labor itself.
In order to reach the above conclusion however, Marx needs to assume that there eixts such a thing as abstract labor, labor in general, that is the common denominator of all commodities. The labor theory of value stands on the notion of abstract labor. We can bring in non-human inputs (nature) to complete Marx's picture (Marx endorsed William Petty's idea that if labor is the father of all commodities, the Earth is the mother). Marx did not see the need to distinguish between labor in its physical aspect and labor in its mental aspect. But we may recognize that just as all commodities have labor embodied in them, so do all commodities have knowledge embodied in them. Making this distinction allows us to explore the meaning of the term "exploitation of knowledge." I will consider two dimension of this in brief.
First, direct (living) labor is not the only source of embodied knowledge. Knowledge can be a non-labor (i.e. capital) input. To focus this discussion imagine the mass production of a certain type of decorative carpet. The inputs to the production of the carpet are the raw materials (fabric etc), the machines needed for weaving and embroidering, electricity, physical infrastructure, and of course the labor needed to operate the machinery, to manage, to sell it and so on. For now, we will keep aside the question of whether managerial and sales labor is exploited as well. But we have not mentoined one more input here: the design on the carpet. Let us imagine two scenarios. First, a designer hired by the firm produces a design which is implemented by the workers on the shop-floor. Second, a manager spots a certain type of artisanal design in the market, which happens to be unprotected by patent or copyright. He buy the artisanally produced carpet, takes it back to the firm and starts mass-producing a similar (or as close as he can get) product. Both scenarios are fairly common. In the first scenario, the knowledge of the designer is exploited in the form of his labor, just as the knowledge of the shop-floor workers is exploited via their labor. In the second scenario, what exactly is happening? We can assume that chancing upon a good design was a good deal less costly for the firm than paying someone to produce it. In fact, designs in the public domain are often used for this very cost-saving purpose. But was this design in the public domain? This question of course leads up the thorny question of protection of lokavidya. This is an important issue but one that I want to side-step for now. Let us say that this lokavidya (the carpet design in this instance) was indeed unprotected and therefore de facto "open access." Was the artisan whose design it was (or the aritsan community if it was a communal deisgn) exploited because their input created value, but they received no return for it? If this is exploitation it is exploitation of knowledge as distinct from exploitation of labor. Instances of this can of course be multiplied ((particualry well known examples exist in the bio-pharmaceutical industry). I have not even brought in the complication that the carpets produced by the firm may compete with artisanal products and destroy the livelihood of the very source of the design.
Second, there is a further complicating factor: just as the labels "skilled" and "unskilled" attached to labor are not merely objective descriptions of the labor process but rather political weapons in the fight against labor, the difference in respect accorded to various types of knowledges, performs a similar function, allowing greater exploitation of certain types of knowledge. We all sense the dichotomy between school/university (formal) knowledge and on-the-job/informal knowledge (lokavidya). The lack of respect accorded to lokavidya makes possible greater extraction of value since the socially accepted standard of living(which is an important determinant of the cost of necessary labor, i.e. the wage) differs greatly between formal and informal economy workers. Since an aritsan is socially not expected to ("does not deserve") the same standard of living, it follows that he/she can be paid less than an engineer who may perform similar functions. Here it may be argued that if informal economy workers are paid less, then thier products also sell for less, making their rate of exploitation similar to formal workers (who earn more and whose product sell for more).
This question cannot be answered in the abstract. The informal economy produces a very diverse range of commodities, not all of which are low-quality consumer gooods for the poor. Certain informal (artisanal) products in fact sell for a lot of money of which a very very small fraction reaches the producer. Maureen Liebl and Thirthanker Roy (Handmade in India, in "Poor People's Knowledge", a World Bank Publication) give one example. A certain type of decorative brass globe made in Jodhpur, Rajasthan, may pay the producer $1 per piece. The same piece sells for around $70 in New York. This is not really news. But I suspect that this type of exploitative situation prevails not only in high-end artisanal commodities but also in more run-of-the-mill products like garments, furniture etc. If the connection has to be made from repsect accorded to lokavidya (or various types of vidya) and exploitation of knowledge, more works needs to be done in this area.
Finally I note that exploitation of knowledge as defined here is not at all a new phenomenon. Perhaps it is as old as exploitation of labor itself. However, the rhetoric of the post-industrial knowledge economy accompanied by the new "discovery" of traditional and indigenous knowledges, the unsettling of the hegemony enjoyed by modern science, together conspire to create conditions wherein the knowledge dimension of exploitation of the poor can be made obvious and furthermore, the clarification of this issue can make the case for just economic returns even stronger.