Marvel's last contract year is the 2008 season. His 2008 salary is $3.95 million, with an overall cap hit of $6.645 million. His pre June 1 cap hit, if cut, would be $2.195 million, post June 1st it would be $2.695 million.
Given his declining play, does it now make sense to cut him and use those savings for another lineman? I am not saying that we should necessarily throw money at Starks (who has played well) - especially given Starks' track record (as well as the stupid coin we threw at Simmons), but at this point cutting Marvel makes an increasing amount of sense.
Unless you have someone in mind, though, you're only one injury away from Trai Essex.
I'd think that you want to fix the IOL first. Faneca could well be gone, and Mahan-Simmons in tandem has often been pretty grim. Using Marvel's cap savings to address this might help, but also leaves the Steelers thin upfront.