Ask HN: Hard caps on AWS spending?

Hello! I want to try out AWS, but am always afraid of potentially getting an unexpectedly high bill.

I'm the kind of guy who pays some extra for unmetered hosting, just in case, even though I know it's not necessary. It looks like I'm not alone, though [1].

I was wondering, since AWS is big in HN, if any of you knew of a way to instruct AWS to, say, not exceed by any means $100 monthly.

Thanks in advance for your replies.

[1] https://forums.aws.amazon.com/message.jspa?messageID=310174