It's not a percentage change, it's a structural change so it depends on how you use Cloud Datastore (largely around indexes). There is an example on the pricing page near the bottom: https://cloud.google.com/datastore/docs/pricing

"For example, in the current pricing, writing a new entity with 1 indexed property would cost 4 write operations. In the new pricing, it would cost only 1 entity write. Similarly, deleting this entity in the current pricing would cost 4 write operations, but in the new pricing it would cost only 1 entity delete."