The built-in cost of providing an interment space includes digging and filling the grave, but it also includes other cemetery administration services.
Some companies add a "recording fee" independent of opening-and-closing. On example is a three-tiered recording fee, escalating in cost $ depending on the intricacy level of the research needed before opening (and closing) a grave.
In a 100+ year-old cemetery a lot can happen over the years besides deaths: divorces, marriages, adoptions, un-adoptions, bequeathals, un-bequeathals, name-changes, location transfer, rule changes... so before cemetery managers meet with a family all research has to be completed; hence, the "recording fee".
Opening-and-closing also includes the attendant services and accessories: installing the burial container, setting up the lowering device, setting up and taking down whatever form of shelter the cemetery uses, chairs, any little ameneties like bottled water, kleenex, etc. In the case of bigger, heavier containers many cemeteries charge an additional fee to install the container.