I've never come across a list of average board scores but I'm sure there's one out there. I imagine top-tier schools have higher averages, as expected. Why do you think this is important though? There are so many other ways to differentiate between medical schools, many of which are very personal, making certain schools good "for you" and perhaps not as good for others. The unusual curriculum at Duke is a great example; I absolutely LOVE it, but it's definitely not for everyone. What I'm saying is, I'd be VERY cautious about choosing a medical school on the basis of USMLE score averages, or even using it to somehow "inform" a decision. Think of it this way, if you're unhappy for your first two years of medical school, does a good Step 1 score really make up for being miserable? And then you'd still have two years left!
I don't mean to be rude, I just don't see how this type of question will be productive in any meaningful way in informing your choice of medical schools. Spend your time talking to students who actually live and learn at these places, and ask lots of tough questions on interview days...THAT'S how you'll figure out where you'll be happy.