Cost-effective might not be the best description. If the B2 is able to target SAMs with very low losses, then it could still be cost-effective compared to significant losses of other airframes and crews.
The B-2 does have its place and is better suited for certain jobs, albeit at too high a cost. The B-21 is purported to lower that. We'll see.
Edit: Looks like current B-2 operational/maintenance costs are now down to only about 2x that of the B-52, which is an impressive reduction (no sarcasm).
They get upgraded regularly, the only reason to make more bombers is to try something different.
During the recent engine replacement project using one full sized engine per boom instead of the twin small engine nacelle was seriously considered. So 4 engines instead of 8. I suspect the reason the twin nacelle was kept was that going to 4 engines required more engineering rework than they were happy with. It certainly would have improved the b-52's range and fuel efficiency.
Either way probably a net zero on the stealth consideration.
Fun fact: with the re-engine project the b-52 now has the same engines as the A-10.
Do we have this class of anti-radiation weapon?
If the radar is mobile, that means that you need a true anti-radiation missile, but SAM radars aren't mobile, are they?
Also only the radar really transmits, so even if you hit it the, separate launchers might still be intact & guided by another radar if you don't destroy them.
This is why modern remotely guided middle strike drones are so dangerous for SAMs - they can basically search an area for all the SAM system components and destroy them all or most of them. Potentially much more effective than a homing missile hitting just the radar at maximum.