Further to the notion that you cannot claim "all" of the savings afforded by an invention, take a look at the Mobil/Amoco case of 1994. In that instance Mobil wasn't even trying for the whole amount - they tried to claim 1/2 the savings to Amoco as a "reasonable" royalty. Amoco used the existing agreements Mobil had in place to show that the number being asked (the "savings" calculus) was 8 times too high.
So I don't know if Rambus is going to go this route, but I suspect they would not get far, particularly if the existing agreements are significantly below the "savings" figure.
Da Greek