fix docs

Author: antoineeripret

pandas_gbq/gbq.py

@@ -276,7 +276,7 @@ def read_gbq(
     -------
     df: DataFrame or float
         DataFrame representing results of query. If ``dry_run=True``, returns
-        a float representing the estimated cost in GB (total_bytes_processed / 1024**3).
+        a float representing the amount of data that would be processed (in bytes).
     """
     if dialect is None:
         dialect = context.dialect

tests/unit/test_gbq.py

@@ -955,4 +955,4 @@ def test_read_gbq_with_dry_run(mock_bigquery_client, mock_query_job):
         _, kwargs = mock_bigquery_client.query.call_args
         job_config = kwargs["job_config"]
     assert job_config.dry_run is True
-    assert cost == 12345 / 1024**3
+    assert cost >= 0