I once got 10,000 Imm xp for one reward. Them were the good old days. Maybe the decline of amount was when new edges came in. They don't seem too far off (rewards are scaled to match them now)

Sun Oct 8 11:41:30 2006 by 'An Immortal' at level 51 (158 hrs):
An Immortal added 10000 exp for: Cool RP and the first Nexus mage again yo!