The problem is that a billing alert at $10 won't prevent you from accidentally starting up something that will bring you a $1000 bill before you can react to that email - there needs to be a process that cuts off service at some hard limit instead of just sending an email and continuing to spend money.
I run a small server and have a few odds and ends and get emails at $5, $15, $50, $75, and $100. Haven't broken $75 limit yet...