const myJson = `{
      "11": {
        "name": "One",
        "cost": 10000,
        "description": "Bla"
      },
      "22": {
        "name": "Two",
        "cost": 11000,
        "description": "Bla Bla"
      },
      "33": {
        "name": "Three",
        "cost": 12000,
        "description": "Bla Bla Bla"
      }

    }`;

    const keys = Object.keys(JSON.parse(myJson)); //массив ключей

    $('.box .item').each((index, item) => {
      $(item).attr('data-id', keys[index]);
    });