Dak Prescott is a good NFL quarterback, that is hard to deny whether you believe he is in the upper tier of quarterbacks or more in the middle of the league,1856 yards and nine touchdowns in just five starts was an impressive start to a season cut short by injury against the NY Giants.
That being said, paying a quarterback when you are not sure if he is a guy that can lead you to a Super Bowl is always a risk. The Cowboys were put in a corner, while he may be a bit overpaid, it would have been difficult to find a guy to replace him that is just as good if not better.