OP is right though, realtime alerting is non-trivial to build. It looks us a lot of work at Vercel to get right. We also offer budgeting options where you can set spend limits now, too.
I just had a look at the billing screen and I can see notifications but not budgeting options (on the Pro plan). Ideally I'd like to monitor/configure spend per deployment as some of our sites have big sales which can = big fees. Is this possible?