The appraiser's who do have real data on this subject, those who make a living doing it, do not give it away.
FYI, I once asked an appraiser who specializes in Detrimental Condition's appraisal work what his fee would be for the type of case I was working on, and he said his Retainer would be $6,000.
Think about this, how much time is it going to take you to create a Case Study on this issue, to include Paired Sales that demonstrate percentages from similar DT's?
Pretend you are a quarterback, now is the time to pass or hand-off to another. Better to pass the assignment off. Plead incompetence. I do that all the time. I often tell clients I am incompetent when faced with something new that I have not done before. There is no shame in invoking the Competency Rule.
To gain competence, take the seminar on Detrimental Conditions and read the book on Real Estate Damages and affiliate with someone with true skills.
I had a guy working for me who went to work for Randy Bell,
www.realestatedamages.com at a starting rate of $38/hour as a researcher in 1998. He was happy as a clam, billing out 60-70 hours a week.
This is a guy that when I met him, was working for free for a residential appraiser. He has all the data, he is the keeper of the keys so to speak.
Do you think he will give me data free, No. Will Randy or his partners, No. They sell data and analysis. That is my point. I actually refer stuff to them, rather than try and dig up the data needed to prove stuff myself. Less pressures that way.