I dont really know why there is a cancel percentage of 50%, but lowering that on some of the expensive things like Town halls and wizard towers would be sick.
Paying 2 million for a town hall and canceling it to only get 1 million back is kind of crazy.
But if there is a reason why i there is a percent i could understand
If there was no percentage I would upgrade my townhall every night before I go to bed and cancel it every morning when I wake up to protect my loot. That's why it's a percentage.