What cost? Unless you are a VIP flyer, the tickets are generally not refundable. And besides, this was not an overbooking situation.
UA could have offered $1000 cash + re-booking and this would never had happened.
“What cost”?
Let me explain this to you. If you have a 100 seats, and you sell 110 at $200, then you have $22,000.
If from now on you have to sell 100 because of this type of behavior, even if not overbooking, in case you have to accommodate your employees, etc (exactly this situation), then you get $20,000.
The $2000 loss gets eaten by future customers in more expensive airline tickets. What other explanations would you like?