[snowwrestler]

Adjust the way you think of the word "scope."

To people who build websites, scope means technical scope--the features, functionality, design, backend etc. A precise technical scope makes it easy to calculate a price.

But to people who buy websites, scope means financial scope--a "$100,000 project" vs. a "$20,000 project." They don't know precisely what they want...because they don't know what is possible. That's why they are hiring you!

So you learn as much as you can in the RFP process and propose your fixed price based on that. Unless the client or RFP is a terrible lie, you should be able to get it in the ballpark.

From there, closing the sale depends on your ability to convey that your expertise will help the client get the best possible product for this "scope" (i.e. price). If you want to cover a few more bases, you can provide some optional levels of effort for the riskiest items, or "nice to haves."