If you really wanted, you could create a script that after a certain billed amount gets reached switches your site via route53 over to a static s3 page that says "down for maintenance" or something until you figure out why your forecasted billing amount is so high. forcastedSpend is an object you can call via api: http://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/...
EDIT: I've reposted my question as a top-level comment to give it a bit more visibility as I'm interested in seeing answers as to what would be a fair price for such a service.