There is no way of knowing if it will be or not. The player could go on and reach his potential and then they sell him for £30m, which would make turning down £15m look like good business. Equally, he could not fulfill his potential and they sell him in a few years for £3m, which would make turning down £15m bad business.
An example is Hudson-Odoi, Chelsea turned down £70m from Bayern one season, and then a few years later sold him for £3m to Forest.