But that is what they signed him for and the role he was signed to fill. And comparing top OTs to top OGs, the contract difference is around 3 million. (I think the NO OG and Mankins are paid about 8 mil/year and Jake Long about 11 mil). So if there is a 3 mil difference between the value of OGs and OTs, Levi's worth would be around 6 million in the open market.
Jake Long's finishing up his rookie deal. I have no idea what the real market for offensive tackles are right now. Joe Thomas just signed a ridiculous contract extension with Cleveland, but Joe Staley just extended in San Francisco, and he's making less than $4 million a year.
With so many young offensive tackles on their rookie contracts around the NFL, and so many others as journeyman types, it's difficult to establish what the market for a mid-tier offensive tackle is. Something less than D'Brickashaw Ferguson but something more than Joe Staley, I'd guess.
Look up Staley's contract on Spotrac.com and tell me if you can make sense of why he'd sign on to that deal.