Since the recent patch, level 80s have noticed gold earnings reduced from quest rewards. Blizzard recently took out the additional experience reward to level 80s, in preparation for the upcoming Cataclysm release.
In other words, you still get the gold reward from the quest, just like someone below level 80 would, but the added experience converted to gold is no longer there.
The reasoning is to prevent level 80s from completing a lot of quests, keeping them in their log, and turning in immediately after Cataclysm launches and the level cap goes to 85 for a quick experience jump.
