Ask HN: How large a portion of your team's cloud bill is caused by databases?

I'm consulting with a team where the database costs comprise ~90% of the entire cloud bill. This seems like a ridiculous ratio, but I haven't had access to such billing information before. Is it common for database costs to balloon like this? I wouldn't consider the team's treatment of the database to be good, but this cost is insane. Have you experienced anything similar, and how do you even go about optimizing a database that's been driven into the ground?