python中json数据的聚合和求和

我是python新手,使用python3。我有json数据,比如:

{
    "message": {
        "count": 46,
        "limit": 1000,
        "schools": [
            {
                "class": "1",
                "class_id": "1c8***",
                "charges": [
                    {
                        "cost": 10,
                        "breakdown": [
                            {
                                "books": "1",
                                "unitQuantity": "10"
                            }
                        ]
                    }
                ],
                "area": "maccau"
            },
            {
                "class": "2",
                "class_id": "1c3***",
                "charges": [
                    {
                        "cost": 100,
                        "breakdown": [
                            {
                                "books": "1",
                                "unitQuantity": "100"
                            }
                        ]
                    }
                ],
                "area": "maccau"
            },
            {
                "class": "1",
                "class_id": "1c3***",
                "charges": [
                    {
                        "cost": 10,
                        "breakdown": [
                            {
                                "books": "1",
                                "unitQuantity": "10"
                            }
                        ]
                    }
                ],
                "area": "maccau"
            },
            {
                "class": "2",
                "class_id": "1c8***",
                "charges": [
                    {
                        "cost": 50,
                        "breakdown": [
                            {
                                "books": "1",
                                "unitQuantity": "50"
                            }
                        ]
                    }
                ],
                "area": "maccau"
            }
        ],
        "url": {
            "link": "/"
        }
    }
}

我可以使用json.loads加载数据,并尝试获得如下结果:

class   Cost
1       20
2       150

我试着把json转换成字典:

item_dict = json.load(json_data)

尝试使用for循环获取数据,检查类是否为1,然后汇总成本。但我觉得这不是最好的办法。有人能告诉我最好的办法是什么吗?