重み付きランダム ref: http://qiita.com/divideby_zero/items/a8e749e307013ab24a0b

file0.cs

public class MyUtil
{
    //渡された重み付け配列からIndexを得る
    public static int GetRandomIndex(params int[] weightTable)
    {
        var totalWeight = weightTable.Sum();
        var value = Random.Range(1, totalWeight + 1);
        var retIndex = -1;
        for (var i = 0; i < weightTable.Length; ++i)
        {
            if (weightTable[i] >= value)
            {
                retIndex = i;
                break;
            }
            value -= weightTable[i];
        }
        return retIndex;
    }
}

file1.cs

    public void Start()
    {
        var weightTable = new int[]{
            1000,
            50,
            1
        };

        int index1 = MyUtil.GetRandomIndex(1000, 50, 1);
        Debug.Log("index1:" + index1);

        int index2 = MyUtil.GetRandomIndex(weightTable);
        Debug.Log("index2:" + index2);
    }

file2.cs

        var count = new int[3];
        for (int i = 0; i < 100000; ++i)
        {
            var index = MyUtil.GetRandomIndex(1000,50,1);
            count[index]++;
        }

        for (int index = 0; index < count.Length; index++)
        {
            Debug.Log(index + ":" + count[index]);
        }

file3.cs

        var weightTable = new int[]{1000,50,1};

        var count = new int[3];
        for (int i = 0; i < 500; ++i)
        {
            var index = MyUtil.GetRandomIndex(weightTable);
            count[index]++;
            weightTable[index]--;
        }

        for (int index = 0; index < count.Length; index++)
        {
            Debug.Log("取得" + index + ":" + count[index]);
        }

        for (int index = 0; index < weightTable.Length; index++)
        {
            Debug.Log("残り" + index + ":" + weightTable[index]);
        }