FREE MultiplyBuildingCost : 1.0.0 1.0.0

using System.Collections.Generic;
using Oxide.Core;
using Rust;

namespace Oxide.Plugins
{
    [Info("MultiplyBuildingCost", "YourName", "1.0.0")]
    [Description("Multiplies the building cost by 3x")]

    public class MultiplyBuildingCost : RustPlugin
    {
        private void OnServerInitialized()
        {
            ModifyBuildingCosts();
        }

        private void ModifyBuildingCosts()
        {
            var itemDefinitions = ItemManager.GetItemDefinitions();

            foreach (var itemDefinition in itemDefinitions)
            {
                if (itemDefinition.category == ItemCategory.Resources && itemDefinition.GetComponent<ItemModConsumable>() != null)
                {
                    // Modify building block costs
                    var buildingBlock = PrefabAttribute.server.Find(itemDefinition.itemid) as BuildingBlock;

                    if (buildingBlock != null)
                    {
                        buildingBlock.grades.ForEach(grade =>
                        {
                            grade.costToBuild = (int)(grade.costToBuild * 3f);
                        });
                    }
                }
            }
        }
    }
}