// generate a topic to use
var topic = new Topic(...);
topic.AddSubscription(new EmailSubscription("me@nowhere.com"));

// now use the low level apis to create a budget with alerts
var budget = new CfnBudget(scope, "MyBudget", new CfnBudgetProps
{
    // define a monthly budget of $10.00
    Budget = new CfnBudget.BudgetDataProperty
    {
        BudgetLimit = new CfnBudget.SpendProperty {Unit = "USD", Amount = 10.00},
        BudgetName = $"Account Monthly Budget",
        BudgetType = "COST",
        TimeUnit = "MONTHLY",
    },

    // define notification parameters
    NotificationsWithSubscribers = new[]{
        new CfnBudget.NotificationWithSubscribersProperty
        {
            // notifiy when the forecast hits 80% of the budget
            Notification = new CfnBudget.NotificationProperty
            {
                ComparisonOperator = "GREATER_THAN",
                NotificationType = "FORECASTED",
                Threshold = 80.00,
                ThresholdType = "PERCENTAGE"
            },

            // define subscribers
            Subscribers = new[]
            {

                // add the topic from above as a subscriber
                new CfnBudget.SubscriberProperty
                {
                    SubscriptionType = "SNS",
                    Address = props.BillingTopic.TopicArn
                }
            }
        }
    }
});