I'm new to this - what's the best way to prepare my data before migration?

You're absolutely right, and the cost estimate is the only thing that turns a wishlist into a real plan. I'd add that the cost isn't just ingestion. You need to include the ongoing compute cost of running those new queries in the shiny, expensive new platform. A monster report that runs fine on your tuned, on-prem warehouse can easily become a five-figure monthly line item in a modern cloud setup.

The flip side is that the cost exercise forces decommissioning conversations that should have happened years ago. Half of those 45 retired dashboards were probably orphaned by a reorg three years back. Nobody wanted to be the one to turn off the lights until they saw the price tag.