Do you know how long the Dataflow processing will take? You may be able to estimate the length of the full-size process (assuming it's batch rather than streaming) by running on a small data sample to see how many vCPU hours it takes and estimate from there, assuming your data takes a uniform amount of time to process.

Since Dataflow is running code that you wrote, it doesn't know how complex your processing will be for any given value, so it's hard to give any reasonable estimate. In general, automatically predicting how long an arbitrary piece of code will take to run on a given input is even harder than the Halting problem (https://en.wikipedia.org/wiki/Halting_problem), which is already undecidable.

Details on Dataflow pricing: https://cloud.google.com/dataflow/pricing

Pricing calculator to estimate cost for Dataflow and other products: https://cloud.google.com/products/calculator/

Hope this helps!