For example, I would like to know how to calculate the charge of incremental export before I run one.

One blog says it is charged by the amount of change logs (but the official doc doesn't say so), which makes sense. But how do I estimate the amount? My hunch is: Put + Write + (1~100) * TransitWrite + Update + Delete + (1~25) * BatchWrite.

Unfortunately that seems like a question for AWS support. I'm not sure they'd be willing to document a formula.