If United (Or Republic Airline) flew an Embraer 170 (same as 3411) just for the crew that needed transport to Louisville, the cost would be about $5k for a round trip. The E170 costs about $2500 an hour to operate.

Stupidly, The cost of the likely lawsuits (even from lesser incidents, that didn't involve beaten passengers, just extremely annoyed passengers) often negates any cost saving for the airline.