In Missouri, we buy a plate for the truck that is based not only on what the GVW of the truck is, but whatever the GCW ( truck and whatever trailer you pull ).
So for instance, my Frontier could have just a 6000 pound plate if I were not pulling a trailer, but since I am at 8500 combined with the trailer hooked up, I have to buy a 12,000 plate.
From there it goes to 18K lb, and up from there.
The difference between a 6K plate ( $25/year )to a 12K plate ( $38 ) is minimal, so it's really no big deal.
Our one ton dually jumps from $60 for 18K lbs, to $100 for 24K pounds. Since I am regularly at 20K to 22K with the F350, we have a 24K plate.
I believe the thinking is that heavier vehicles do more damage to the roads so they should pay a higher regis fee.