The Business Analyst is expected to gauge which requirements have the potential to affect the estimate and so need to be clarified further, and which will not change the required level of effort if detailed more in order to produce a high-quality budget estimate. The resulting estimate may be declined by the client and never make it into implementation, so it is critically important not to spend time figuring out details we may never need.
The Business Analyst breaks down the high-level requirements into logical, executable tasks, finalizes the specific details, passes them to the implementations/development team, and supports the team until the project is closed.