Start with the jobs customers need to do
List a small set of useful tasks: finding a document, checking status, submitting a request or reviewing billing information. For each task, record who may act and which system holds the information.
Separate interface work from system work
A simple-looking screen can depend on several integrations and account-level permissions. Ask whether the estimate includes authentication, invitation handling, record matching and recovery from an unavailable source.
Account for the first release and ongoing care
The build, migration, hosting, provider subscriptions and maintenance are different costs. Clarify which are included and who owns accounts and credentials after handover.
Make the estimate comparable
Give suppliers the same journeys, data structures and acceptance criteria. Ask them to identify assumptions rather than comparing unexplained totals. A discovery stage can reduce uncertainty when the existing systems are poorly documented.