Alert me when a bill is over a specified confidence interval.

For example, if my bill is $10/month +- $5 (std), then it will alert me if it exceeds $6.49 more than the $10/month (the 80% confidence interval.) This is useful for phone bills which can go up by a certain amount (e.g. long distance) but don't have a fixed threshold. If they're over by a few dollars for a few months then I've probably added an add-on, but there's no "fixed" amount that it should be, just that it's not too much more than it was.