Well...in my days, the difference in reward was: -

No really. You got the exact same things regardless of whether you chose Very Easy or Infernal as difficulty. Or whether you set the difficulty bar to 1 or 100 in games with a slider. Later on (especially on Steam), you got a different achievement for clearing each difficulty, if at all. And that was it.

It's a concept that, for some odd reason, works for millions of single-player games. Modern titles as well. Yet in MMOs, it seems to be unfathomable.