For an Embraer 170 aircraft (70 seats) the operating cost of the flight is about $2500-$3000 for the Chicago to Louisville flight. A supplementary flight with the 4 crew and any re-booked passengers would have cost the airline about $5,000 if they flew out-and-back as a non-scheduled flight.
SWA 737s cost around $4,000/Hr to fly, but a non-scheduled (Out and back) flight on a shorter route would still cost less than $12,000 in operating expenses.
The $1000+ flight credits for bumped passengers add up fast, but supplementary flights are relatively rare for airlines, other than for cancellations due to maintenance issues. CEOs like Oscar Munoz need to make supplementary flights easier to authorize, and the negative PR would be significantly reduced.