So you want to proactively determine if, at the current rate charges are accumulating, the budget will be exceeded?
That _also_ runs into problems!
Take, for example, a nightly job that spins up a few giant instances to do some batch processing and shuts them down. Running an hour a night, over the course of the month that's going to accumulate ~$300 in charges. Great, we can set a $400/mo budget and have some wiggle room and all is well!
But how can AWS know that you're going to shut the instances down? Looking only at the rate charges are accumulating, the first night those instances start up you are on track to run up a $7,000 bill!
So do we set a $400/mo budget and then just kill the account so it stops accumulating charges when we hit $400, or do we set a $7,000/mo budget to account for the potential rate of accumulation and risk exceeding our budget by 2,000%?
It would be nice if this were in fact just overcomplicating things, but after much thought and many arguments on the internet I really can't see an easy "general" solution to this. The solution is heavily dependent on your specific workload and usage patterns, and the tooling is there to manage that if you want: Create billing alerts, and run code to adjust your usage in response to them.
That all said: I would fully support some sort of "developer sandbox" account that allowed a "kill the account" billing limit. I'd really prefer it had some sort of obvious limitation to avoid people accidentally using it for production workloads or dev workloads turning into production ones. Something like a hard limit that shuts the account down in 30 days, or limiting inbound connectivity to only via a VPN or something. That's purely self interest though--I don't want to see the article on the top of HN every few weeks about how "Amazon killed my startup" because someone set a billing limit and then all their customers' data was deleted.