IIRC on international DL tickets exUSA, I've seen $300 and $450 change fees recently, but $800 wouldn't be impossible in some markets, especially originating outside of the USA.

Of course, calling it substantial or not depends on your perspective and one could view a $300 change fee on a cheap TPAC/TATL coach ticket that perhaps only costs $500 to be substantial, while a $450 fee on a D class ticket might seem like almost nothing to someone.