According to the patch notes, class represents the price of the plot as compared to other plots of the same size (small, medium, large). Class 1 will be the most expensive while class 5 will be the cheapest. The class of the plot is determined purely by location. Obviously, beach-front property will cost more than property tucked away in an uninteresting corner.

Before entering the residential areas, you can choose to select which ward (instance) to go to by clicking/selecting "Go to specified ward" and it'll give you a full list of all the plots, their size, and their price.