Carnival Boy is pretty much on target. Sell the sizzle, not the steak. In initial meetings we give high level assessments, and explain the devil is in the details. Like Carnival Boy, we give a ballpark range, and then if they want a hard static quote, we would charge for that analysis.
If it is an RFQ, we always put a disclaimer that if the actual environment is different then the RFQ there would be additional charges. Plus we always pad hard RFQ proposals with a buffer for the unknown.