That is, this is much less concerning if the numbers are such that 90%, say, is proven illicit.
To get the money back, it must be fought in court. Legal fees are not returned if the case is won, and if lost, you must cover the defense fees. In Illinois the median forfeiture was about $1k, with many <$100 forfeitures clustered in the poorest neighborhoods, while lawyer fees are ~$3k for state/local police cases. Federal seizures are much more expensive to fight.
There's no process where the police further verify the provenance of the cash they seize and return it without it being fought for in court. It's taken and they celebrate it and move on. So as with most police oversight, the numbers you're asking for don't exist.