Just a recent data point - upgrade from UA award J to F at check-in was ~$2,000. Went up $100 at the airport. I was not given the option to bid on an upgrade beforehand, but they must have accepted quite a few LHG bids as we were F6 a few days out, but F2 on departure. This was the early BOS-ZRH flight.
You can’t bid on 016 tickets. $2k is a lot for this route IMO.