[QUOTE=canadiancow;27464243]Won't happen.
There is an incremental cost of moving you from Y to J (they may have to cater an extra meal, you will likely consume more beverages, etc.), which would set some floor on it, though it would likely be around $50.
QUOTE]
Yes. I think I added too much personal-experience bias to this quick thought as I tend to only fly at peak travel times when Y is oversold and that incremental cost is incurred as a result of op-ups. I know that some of those op-ups are awarded based on status, but I suspect that a few have gone to the lingering no-status GTEs. In either of those cases, the incremental cost is borne by AC, not the passenger, in exchange for the opportunity to oversell Y without actually leaving any confirmed passenger behind. With an auction, the winning bidder pays at least some of that cost.