This is going to piggyback off the NFC North Blog on ESPN: Click Here
Although the article is about Rodgers, I think it applies to Stafford the same way.
If Stafford was to take below market value for the Lions, there is no way of knowing if the team would use that cap money to help put a better product on the field. The Lions could easily use that cap space to cut any player they think is overpayed, and let the dead money eat up the cap. They also could take the Bucs/Bengals approach, and just not use the cap at all.
Players have a very small amount of time to get payed, I cant blame any of them for asking market value.