What if, after reading the RFP, you're still not sure about the parameters of the project, and you can't get additional information from the prospective client? That's not unusual. Many RFPs are extremely brief and vague -- in fact, an RFP may be issued primarily because a client doesn't really know what it wants and is hoping to get ideas or clarification from the proposals it receives. Some IPs see that as an opportunity to be creative and show the client how the IP thinks the project should be approached. Others take the opposite view, that a vague RFP is an invitation to waste massive amounts of the IP's time trying to read the prospective client's mind. Unfortunately, there are plenty of examples that could back up either view.
At a minimum, you must get from the prospect a definition of the problem they're trying to solve, or what they're trying to accomplish, even if they're clueless as to how to proceed. Holtz suggests proposing a low-cost solution that provides one way to solve the problem, and then including additional items and the cost for implementing each one. In this way, you protect yourself from a project getting beyond your control, and you provide a fair pricing scheme for the client.