Before quoting a migration you need to know how big the system really is. Documentation rarely gets it right.
An example from a railway system
In the analysis of a railway system (client not named), the inventory reached 2,009 objects, 159 tables, 1,238 columns and 155 foreign keys, with 54 composite primary keys. The documentation underestimated the real size of the system.
Why the schema rules
The document describes what someone wanted the system to do. The database schema and the behavior of the system show what it actually does today.
What we do instead
A measured sample: 5 to 10 screens turned into a verifiable specification, with tests and an inventory of differences between the documentation and the system.
What we ask for and what we do not
We ask for help pages or screenshots, the table structure and a functional owner. We do not ask for real data.