Amazon Bedrock bills cache writes at 1.25 times the uncached input rate and cache reads at a 90% discount, per [AWS's caching
That sets a break-even of roughly 0.28 reads per write, so a workload that writes cache and rarely reads it pays more than no caching at all.
One reported agent workload spent about $1,182 on cache writes in four days, around 85% of its total, because the client never sent the fields that make a cache readable.