Insurance companies stop just short of criminal, their lawyers make sure they get as close as possible. Not all insurance companies charge by cc. My former Vulcan 1500 and Goldwing 1500 had rates half of what my EX500 Ninja did. Even though a Ninja 500 is slow compared to real sport bikes, it is in a category that is far more likely to be ridden recklessly than a Goldwing or a cruiser. I really don't see the point in these bikes. My EX500 got me 2 speeding tickets (fortunately no points) There is nowhere in the U.S. you can legally ride more than 75 mph (other than maybe Montana) and the Ninja would easily top 100 mph. They could sell these bikes, but for track use only, the way MX bikes are sold.
And yes I know there are some ridiculously fast cars out there (and their owners also pay ridiculous insurance rates) but I almost never see them driven in a reckless manner, certainly no more than any other car. A couple of weeks ago I was driving to a town about 200 miles away, and it includes over 100 miles of nearly straight road. Speed limit is 75, but I was early, had time to waste, road was almost deserted, so I was going 65 in the right lane. A couple of nearly new Porsche Turbos passed me in the left lane, maybe doing 75-80 as they carefully passed with plenty of room. It could be that someone who is wealthy enough to own such a car is also smart enough to know what they have to lose if they were to cause an accident by reckless driving.
As for the insurance on your Duke, maybe try another company. Not all of them rate bikes the same.
2002 Vulcan 750 (engine out, slowly being disassembled) 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
2012 Zuma 125, 1980 Puch moped