I think that there is no reliable way to get an industry standard CTR.
The main problem is fraud. I have seen sites that coerce people to click with lines like "Help keep our site free -- click on a new banner every day", or with schemes where content gets removed if people don't click on banners. Or people click on their own banners or have "click rings".
I've also seen people cheating with many banners on a page, or banners that are supposed to run on the top of the page buried at the bottom.
Additionally, I see a lot of people placing banners on Chinese-language sites -- not so single out Chinese people, but I have seen at least 5 instances of this where someone creates a "front" page and gets accepted into an ad network, then runs the banners on other sites where people don't even know what the banners say.
With so much fraud, I don't know how any number can be quoted. I'd estimate that 20-30% of banner ads are involved in fraud somehow.