public struct DropItem
{
Transform item; // Здесь мы храним данные о нашей вещи;
int chance;  // Здесь храним шанс выпадения (1-99);
}

using UnityEngine;
using System.Collections;

public class DropItem : MonoBehaviour
{
     public Transform item;
     public int chance;

     public DropItem(Transform _item, int _chance)
     {
         item = _item;
         chance = _chance;
     }

     public void CreateDropItem(Vector3 position)
     {
         var drop_item = Instantiate(item) as Transform;
         drop_item.position = position;
     }
}

public void CheckDrop()
     {
         if (dropList.Length > 0)
         {
             int rnd = (int)Random.Range(0, 100);

             foreach (var item in dropList)
             {
                 if (item.chance < rnd)
                 {
                     item.CreateDropItem(gameObject.transform.position);
                     return;
                 }
             }
         }
     }