What is the cheapest material per ton that one could purchase on our Earth in the 21st century? This could also include the cheapest material per ton to harvest so dirt may seem like the obvious answer but because of the easily harvestable dirt per acre it may not be the most optimal solution.
What problem are you trying to solve? Find the most affordable material for "gravitational potential energy storage with solid masses"
What are you trying to accomplish? Have an extremely long term (think stonehenge) energy storage solution.
restrictions/requirements: The material must be gathered together within the circumference of a 10 mile radius far away from large bodies of water. The cost per ton must be considered including transportation costs or logistical concerns.
research: Hydroenergy storage is already being used by pumping lakes to higher ground artifical lakes but a more exotic approach would be required for areas not containing large quantities of water.