I guess I'm the optimist. While it's highly unlikely they can or even want to keep them all, I don't see such a huge problem in keeping all of their starters if they want to. Yes, there are big contracts involved and CAP to consider, but Lang and Sitton are already being paid like top-10 guards. It's not real likely they will see significant raises on these next contracts at age 30. Someone above mentioned $32mil/4yrs which would actually be very affordable. I'd take them both at that money. They probably won't count any more against the CAP next year than they do this year and might actually come in lower with the right contract structure.
Bak is certainly a starting quality guy though not nearly top-10 so I think he will see a more modest deal probably something in the $6-7mil/yr range. Now, if Spriggs makes him expendable then maybe they let him walk, but I suspect TT will want to try and keep him and see how things play out. He usually doesn't let young talented starters at a premium position walk. Bulaga's injury history says to me that if you let Bak walk and install Spriggs you are right back in the same boat because Bulaga just seems to break down too often. I'd sign Bak, and maybe look to let Bulaga go after 2016 or 2017 if he has more injury filled seasons.
Tretter might move on or he might be viewed as just a versatile back-up and not worthy of much more than a minimum type deal. Barclay and Rotterham are easy minimum deals if you decide you want them back.
With Peppers $10mil likely to come off the CAP after this season there will be CAP room for these guys. Now, you will have to balance keeping all of these guys with whether you want to keep Lacy and at what number, and of course Perry and Jones are also part of the mix. Perry and Jones count $7.2mil against the CAP this year so they may decide to keep one or the other with that money and let the other guy walk.