Yes it does depend very much on your Service Level Agreement (SLA) if you have one. It is possible they are leveraging your work to request proposals from competitors. On the other hand, they may be trying to improve their understanding of the architecture to improve their return on investment (ROI).
As a senior consultant, most of the warehouse projects I have been involved in become an exercise in component integration eventually. More often the components are developed from multiple vendors that are usually competitors. I would typically charge for such a deliverable unless there are specific circumstances that warrant a ‘freebee’ or a somewhat reduced price. Remember, your decision will be setting a president for future relations.
I would recommend you – or someone else - investigate the client’s motivation for the request and act accordingly. A little extra time in customer relationship activity now could pay very large dividends for you in the future.