We haven't seen a single case where we don't end up matching the actual AWS bill. In fact, with a release currently in BETA and rolling out in a week or two, we'll be providing richer data faster than AWS Cost Explorer provides.
You can see a demo of that here (designs still need to be implemented) https://www.loom.com/share/dcb72a921f134e59b19a0dd3d3ab0e2f
At a more general level this may also be one of the most entitled asks I've ever seen in the 12+ years I've been on HN.
If you could underwrite price guarantees and show an insurance company or lender that your figures are right in most cases and that you've never lost over a certain amount, you could really hike the cost of your service and provide an incredible utility across the board.
If you have spare bandwidth and can reliably do this, try building this.
The real opportunity here is to start an AWS insurance business.
Where you provide accident insurance for clients by charging 10% of <some estimate of expected monthly running cost>.
And spread the cost of mishaps over the clients you charge.
The trick is to define that estimate of normal cost. And also to certify that clients meet some minimum expectation of clumsiness.
The requirement is insurance against not exceeding a hard limit. We're talking about a rare event, so the offering (vantage.sh) is not good for this specific usecase if it isn't absolutely foolproof.
We try to help out a bit on this with Vantage which essentially gives you a DigitalOcean-esque view of your AWS costs. The first $2,500 in AWS costs are tracked for free which would seemingly cover your side-projects.
It sounds like you've found your home on DigitalOcean, but I'd be curious if something like Vantage would potentially change your decision to build on AWS? In particular what you mention about runaway bills is something that Vantage sends alerts on in advance. We also show you a full inventory of your AWS resources and what they cost you.