There are a many ways one could attempt a Marxist reading of Jane Austen's Pride and Prejudice. First, one could focus on the general absence of the working class in the novel. Pride and Prejudice exclusively focuses on the world of the upper class, depicting the way wealthy families socialize and form relationships. While Austen certainly satirizes the upper crust of society and all of its foibles, a good Marxist reading should point out that none of the working classes are present, and even the servants, who certainly would be present, are not discussed or talked about.
Another potential Marxist reading could discuss the role of wealth in determining romantic relationships. While the Bennet family is by no means destitute, they are also less wealthy than most of their neighbors, and are likely to be turned out of their home unless their daughters can make a "good" match. A "good" match would be a marriage to a wealthy man, as this is the only means by which the Bennet family would be able to hold onto its estate if Mr. Bennet passed away. As such, it is clear that even the formation of relationships is governed by financial transactions.
Finally, the gender roles in the novel could be incorporated into a Marxist reading. The Bennet daughters are completely dependent upon men for financial support, and they are essentially controlled by the whims of the males in their lives. As such, we can see women in the novel as the base, whereas men are the superstructure, the force that controls and oppresses the masses.