Though we're not quite sure why, gold rewards for daily quests have been cut by 50 percent or more across the board. The 7g40s total we're getting now for level 80 dailies is roughly what we would be getting for completing the quests were we not level-capped, so it's possible that all of the patch 4.0.3 goings-on accidentally flipped some invisible switch saying "all them guys is under the level cap now." But we have no idea!
In the meantime, your pre-Cataclysm daily quest farming income has been cleft in twain. You'll probably be forced into a life of crime just to make ends meet, but it's not your fault. The recession hit us all pretty hard.
UPDATE: Now we're sure why! Blizzard says it's a bug. Thanks, commenters!