I have a problem with the premise of the question - doing cost/benefit analysis of carrying the FDR/CVR doesn't sit well with me. By all means do the analysis on practically every other facet of the operation. The CVR/FDR infrastructure is a very minor expense.
I suspect you might be playing devil's advocate...
There are a plenty of accidents which would have remained a complete or at least partial mystery if a CVR/FDR wasn't carried. Actually, I'd suggest the majority.
How do you make progress without knowing what to improve? In the few examples where there are witnesses, they're notoriously naff at telling investigators the aircraft's speed, angle of attack, thrust setting, aileron position, exhaust gas temperature, etc.
On a more morbid note, families have a deep need to know how and why their loved ones died.
B&S