Amazon CloudWatch allows you to use your projected monthly bill as a metric for alerts and send you notification when it exceeds a specific amount.
Add a couple of dollar figures like $50/mo, $100/mo, $250/mo, $500/mo, and $1,000/mo
If you ever end up with a run away server or accidentally provision many servers instead of 1, you will be happy you set up billing alerts.
For the 5 minutes it will take you it can save you a lots of money! So do it now!