Simply put, a devil’s contract is overwhelmingly powerful—it is essentially like making a wish. Whatever you desire, you can have.
To put it even more simply, if you wanted a nuclear fusion device, although Tang Xiaotian didn't have one on hand, as long as such a device existed in this world, a devil's contract could make it happen and hand it right over to you.
As for the price, compensation was certainly required, but it didn't have to be of equal value.
Chapter 186