Sorry to be so late to reply on this. Since you say you can already cite some case studies, and there isn't a list -- and how could there be? how would you keep something like that current?? -- I think you need to not allow yourself to be fenced in by the language of the question they are asking and you should work on trying to figure out how to communicate in a way that addresses their real concern or objection. It is not at all uncommon for people to ask questions that are framed in a way that suggests a certain type of answer -- "yes" or "no", a number, etc -- but it is also very common that such questions are not really answered very well if you accept the framework they presume for the answer. You see this all the time in court room dramas on TV: the lawyer for each side asks a series of questions intending to paint the picture a particular way. Often, the person being questioned does not want to answer "yes" or "no" -- it just isn't that simple!
Since it seems to be a common question, you might want to come up with some resources, try to start a dialog or do a survey to determine what they are REALLY trying to find out (a "number" is probably not really it), or something like that. Numbers are not the only measure of something and may not even be a good measure. There are different rubrics that can be applied to various things. And I also see no reason why you can't honestly say "This is so cutting edge, that comprehensive statistics do not exist for it. It is the wave of the future. I would be happy to refer you to some case studies, but a precise answer does not exist to the question you have asked. If you want numbers, I would be happy to talk to you about cost-benefit analyses that do exist.....blah blah blah"
People usually ask questions like that because they have No Clue (and know how much they are inthe dark and definitely do not like it) and they are looking for some "simple" means by which to judge a complex decision: should we or shouldn't we? What can you tell them which will put their mind at ease? (Does that make any sense??)