My guess is that a true answer is probably as clear as mud.
SDI had a lot of other stuff that they were looking at besides traditional ground to missile interceptors. Directed energy, kinetic stuff, satellite engagement--Not sure anyone could possibly segregate the costs.
The directed energy stuff has been going on for a long time-EdwardTeller was a huge proponent of that in the 60s, and that's where Reagan got his ideas from. I have no clue how far directed energy weapons are away from being a true reality outside of structured and limited testing.
Missile defense systems were proposed back in WW2. Allies were able to shoot down a ton of V1 with a barage of artillery, but couldn't lay a glove on the V2.
My hunch is that the SDI program likely advanced the ball in the development of some of our current systems, but I think the underlying conclusion with SDI as conceptualized, is that we were decades away from some of the technology.
The age of the manueverable hypersonic missile is upon as well. I do not know the success rate of shooting down such missiles in Ukraine. It is more than zero and less than 100% --clear as mud.
My general thought is that SDI took such a wide scope approach to research that it became a jack of all trades and a master of none, but likely generated some level of technology we use today. How much? I have no clue. I don't think boondoggle is the wrong word--good things can happen out of boondoggles (ask
@mcmurtry66 about the girl from room 230 at his last CLE conference

). It seems to me that some significant discussions with industry leaders in this type of field might lead to a beneficial end product that could justify certain costs. Just my 2 cents.