You can't measure the value of a CB against the value of a Guard or Center.
Besides, if you do, you have no argument. CB ranks up there with QB and LT as among the most valuable positions.
Guard and Center towards the bottom end.
This is why top CBs make about twice the money as an equivalent Guard or Center.
Besides, it is not an issue of one or the other; you need both. Who, exactly, can the Rams get that is as good as TruJ? A rookie 2nd round draft pick? Please. Might there be a FA CB that fits Phillips' system better? Perhaps, and if so THEN it might make sense to pursue that CB instead of TruJ. But you're going to overpay no matter who you sign in FA.
CBs are expensive. There's no way out. Even above average CBs cost a lot.
Will they keep TruJ? Depends on his contract demands. If he wants JJ money, probably not. If he'll take something like 4 years at $50 million, yes.