Scale crystal rewards for a match up by the difference in player deviation from average GP in that league/division. Don't ever scale down rewards, but reward players who "punch up".
For instance, the GP of my last opponent was almost 9.5M. My opponents deviation is .28
My GP is only 7.5M. My deviation from the avg in K2 is -2.24.
For winning, I should receive 2.52x the usual crystal payout.