I’ve seen startups with a thousand active users paying $50k/month (though that’s overall costs, not just db). It’s really easy to waste a lot of money doing nothing.
As a small business owner — I recently spent an hour canceling things that just added up over time and that I don’t now need. It’s just so easy to waste money, period.
Directly to your point though, I once encountered a salesperson who was running an entire sandbox environment of a very large platform to the tune of about $25k/mo. It sat idle for almost half a year before someone came knocking. The cloud team did an audit and they were a little spicy about it, understandably.
It’s especially easy to waste money on databases.
People just throw more compute power (ie money) at performance problems, rather than fixing their queries or making better use of indices.