from datetime import datetime, timedelta
import boto3

today = datetime.today()
yesterday = today - timedelta(days=1)
seconds_in_1day = 86400
dimensions = [{"Name": "Currency", "Value": "USD"}]

client = boto3.client('cloudwatch', region_name='us-east-1')
res = client.get_metric_statistics(
    Namespace='AWS/Billing',
    MetricName='EstimatedCharges',
    StartTime=yesterday,
    EndTime=today,
    Period=seconds_in_1day,
    Statistics=["Average"],
    Dimensions=dimensions)
print('$%s' % res['Datapoints'][0]['Average'])