Defensive end Cliff Avril told Pro Football Talk on the NBC Sports Network his chances of returning to the Lions are "50-50."
I think that's a pretty accurate number.

Cliff Avril: Not a given he will be leaving Detroit

There is a common misconception about Avril that he he didn't have a good year in 2012. Actually, he maintained a relatively high level of play even after much of the Lions' defense was falling apart around him. He does want to stay in Detroit, but understands the business aspect of the game, which in the NFL is to make money while you can quickly, because there are no tomorrows guaranteed because of the sport's rugged nature.
He did get $10.6 million for last season carrying the franchise tag. So Avril has already made some big cash.
The Lions can back-load a contract for him with a hefty signing bonus in order to spread the salary cap number over the long term (he is just 26). It would not preclude the Lions from drafting a defensive end with the fifth overall pick in the first round. It would only make the notion of more plausible because bookend, high-level defensive ends would greatly benefit their flagging secondary because of the pass rush aspect.
And Avril would benefit from staying here because he is going to be single-teamed most of the time because of the attention opposing offenses must pay to Lions' defensive tackle Ndamukong Suh.
So while the idea Avril would not be returning seemed likely when last season ended, as the free agent period approaches, there is a possibility he will stay. Both parties might fit each other after all.