By the way, SLS suffers from "Lego Rocket Syndrome", AKA "DIRECT syndrome", in how they plan to start out with a 3 x SSME boattail, and then develop an all new 5 x SSME base for the evolved 130 ton vehicle.
And just maybe, maybe
replace the SRBs with something else down the line.
Somehow this will all be done cheaply and not suffer unexpected cost overruns.
Providing a reality check; the closest we ever got to an actual
Lego Rocket; the INT-20 (S-IC/S-IVB) and INT-21 (S-IC/S-II) variants of the Saturn V, which mixed and matched already developed and completed stages would have cost the following in DDT&E/R&D costs at a minimum
(reality likely would have seen costs go up):
$2.1 billion for the INT-20
$2.49 billion for the INT-21
Completing the cancelled Titan IIIM and finishing up it's man-rating to allow lightweight CSMs to be launched into orbit would have cost $1.3 billion.
Space Shuttle SRB DDT&E/R&D costs from 1970-1978 were $1.158 billion, while SSME development costs from 1970-1978 were $3.1 billion.
(All dollar costs are in 2010 dollars, by the way).