I can tell in your post you're extremely excited about the potential of this client. Keep the first meeting to fact finding and gathering their requirements - what are the must have and what the nice to have the client wants. Keep in mind you also need to see if they are a fit for your company too. If this is a valuable client meaning a case study then do this one for free. The social proof/case study will go further in future business than charging for this one. If the niche is a fit for your company and you enjoy working with these clients, you can move forward and charge on a per project basis or a fixed cost as you will have your timelines/costs defined having a few of these agents already built. You can also repurpose your agent framework and add the delta for added profit.