要计算BigQuery单列更新的计费字节，你可以使用以下代码示例：

from google.cloud import bigquery

# 设置BigQuery客户端
client = bigquery.Client()

# 定义要更新的表的信息
project_id = 'your-project-id'
dataset_id = 'your-dataset-id'
table_id = 'your-table-id'
column_name = 'your-column-name'

# 定义要更新的行的信息
update_value = 'your-update-value'
condition_column = 'your-condition-column'
condition_value = 'your-condition-value'

# 查询要更新的列的当前字节大小
query = f"""
    SELECT
        SUM(BYTES({column_name})) AS current_bytes
    FROM
        `{project_id}.{dataset_id}.{table_id}`
    WHERE
        {condition_column} = {condition_value}
"""

query_job = client.query(query)
result = query_job.result()
current_bytes = list(result)[0]['current_bytes']

# 更新列的值
update_query = f"""
    UPDATE
        `{project_id}.{dataset_id}.{table_id}`
    SET
        {column_name} = {update_value}
    WHERE
        {condition_column} = {condition_value}
"""

update_job = client.query(update_query)
update_job.result()

# 查询更新后列的字节大小
query_after_update = f"""
    SELECT
        SUM(BYTES({column_name})) AS updated_bytes
    FROM
        `{project_id}.{dataset_id}.{table_id}`
    WHERE
        {condition_column} = {condition_value}
"""

query_after_update_job = client.query(query_after_update)
result_after_update = query_after_update_job.result()
updated_bytes = list(result_after_update)[0]['updated_bytes']

# 打印计费字节差异
print(f"计费字节差异: {updated_bytes - current_bytes}")

请确保将代码示例中的your-project-id，your-dataset-id，your-table-id，your-column-name，your-update-value，your-condition-column和your-condition-value替换为你的实际信息。

这段代码首先查询要更新的列的当前字节大小，然后执行更新操作，最后查询更新后列的字节大小。通过计算两次查询结果的差异，可以得到单列更新操作的计费字节。