Oh, I’m pretty sure it’s close to trivial. $0.0004 per thousand requests is $400 per billion, or $0.40 per million.
That’s as close to insignificant as you can get and still pay attention to. Caching solutions are probably going to end up costing you more in the long run. An HA setup that can handle a billion requests a year is going to cost you at least $100 a month, and still provide less availability than s3.

You don’t want unmetered access, but their pricing is unlikely to be based on access rates, and more likely on salary costs and other infrastructure costs, like indexing and search.