Because aws has no billing limit. It is importent to get notified when the cost are growing.
To create an alert you simply go to your admin console and navigate to Billing. There you will finde the point Budget. Here can you define an alert for your account.
First thing you have to do is define the period in there your budget work. Mostly monthly is the choise. Now you have to setup an start end end date. If you leave the end date blank it will be run from start forever.
In the next point you have to define the budget self. Simple enter the money what do you want to spent in a month for aws.
Optional you an create an alert right from the budget.
There one example that send an alert when more than 90% of your budget is used. Notify me when ‘actual’ costs are ‘greather than’ ‘90’ ‘%’ of budgeted amount
Enter your eMail address your for mobile number or where you want to have the alert.