The main point of charging anything at all for a plot would be to finance minimal record keeping: which plots are supposed to be full, who's in them, and, ideally, who bought the space. Plus a quick "do you have a death certificate?" check, and a request to inform whoever's doing the admin if you get to the pauper's field and find the plot you expected to use already occupied. Not an insurmountable barrier for a determined murderer who's done some advance planning, but it should make it less attractive as a dumping ground.