Brandon Weeden was a good gamble at backup QB because he had a lot of physical talent and his salary was minimal. Dallas only paid the minimum with Cleveland paying the bulk of his original deal. That gamble failed.
Cassel was the best of a bad lot of gambles you could take in midseason, when the team needed a guy with experience who could be acquired at midseason and was a guy that could hopefully pick up the offense quickly. That gamble failed as well.
Sanchez is insurance, he has the experience and will have time to learn the system. You have to overpay to have the insurance of an experienced, non-horrible QB as your 3rd QB but its insurance the team should pay this season.