Which features look cheap in planning but become expensive after launch?

I’ve been noticing a pattern in app planning conversations. A lot of features sound cheap because the first question is usually, How long will it take to build?

But that misses the second question: What will it cost to keep running once users start using it?

Things like:

• OTP / SMS login
• Maps and location lookups
• Payment flows
• Notifications
• AI-assisted features

often seem small during planning, but the running costs can end up mattering more than the build cost over time.

Have others seen the same? Which feature looked inexpensive at first but became much more costly after launch? Was it because of third-party pricing, support overhead, infra, abuse, or something else?

Would love real examples.