I think upgrading an OS for many people is more tied to hardware changes than anything else. I wouldn't pay anything to upgrade Windows on a computer that will have its motherboard replaced in the future as the upgrade money is wasted on older technology. This is indicated by slowdowns in computer sales prior to a new Windows release hitting the market.
For me to upgrade a computer I am currently using, the cost would have to be under $25USD. Even then choosing to upgrade would depend on when I had planned to swap motherboards or buy a new laptop.